drm/panel: Add prepare_prev_first flag to Visionox VTDR6130

  Due to a recent introduction of the pre_enable_prev_first bridge flag [1],
the panel driver will be probed before the DSI is enabled, causing the
DCS commands to fail to send.

Ensure that DSI is enabled before panel probe by setting the
prepare_prev_first flag for the panel.

[1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")

Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel driver")
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
---
 drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
 1 file changed, 1 insertion(+)


---
base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f

Best regards,
  

Hi,

On 26/07/2023 00:56, Jessica Zhang wrote:
> Due to a recent introduction of the pre_enable_prev_first bridge flag [1],
> the panel driver will be probed before the DSI is enabled, causing the
> DCS commands to fail to send.
> 
> Ensure that DSI is enabled before panel probe by setting the
> prepare_prev_first flag for the panel.

Well this is specific to MSM DSI driver, it's not related at all to the panel.

Neil

> 
> [1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")
> 
> Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel driver")
> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
> ---
>   drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
> index bb0dfd86ea67..e1363e128e7e 100644
> --- a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
> +++ b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
> @@ -296,6 +296,7 @@ static int visionox_vtdr6130_probe(struct mipi_dsi_device *dsi)
>   	dsi->format = MIPI_DSI_FMT_RGB888;
>   	dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_NO_EOT_PACKET |
>   			  MIPI_DSI_CLOCK_NON_CONTINUOUS;
> +	ctx->panel.prepare_prev_first = true;
>   
>   	drm_panel_init(&ctx->panel, dev, &visionox_vtdr6130_panel_funcs,
>   		       DRM_MODE_CONNECTOR_DSI);
> 
> ---
> base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
> change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f
> 
> Best regards,
  

On 7/31/2023 6:00 AM, Neil Armstrong wrote:
> Hi,
> 
> On 26/07/2023 00:56, Jessica Zhang wrote:
>> Due to a recent introduction of the pre_enable_prev_first bridge flag 
>> [1],
>> the panel driver will be probed before the DSI is enabled, causing the
>> DCS commands to fail to send.
>>
>> Ensure that DSI is enabled before panel probe by setting the
>> prepare_prev_first flag for the panel.
> 
> Well this is specific to MSM DSI driver, it's not related at all to the 
> panel.

Hi Neil,

I think there might be some confusion caused by the commit message -- 
instead of "enabled before panel probe", it should be "enabled before 
panel pre_enable()" as the panel on commands are sent during prepare(), 
which is matched to bridge pre_enable().

IIRC the general rule is that the panel driver should set the 
prepare_prev_first flag if the on commands are sent during pre_enable(), 
so I'll keep the code change but correct the commit message if that's ok 
with you.

Thanks,

Jessica Zhang

> 
> Neil
> 
>>
>> [1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first 
>> to alter bridge init order")
>>
>> Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel driver")
>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>> ---
>>   drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c 
>> b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>> index bb0dfd86ea67..e1363e128e7e 100644
>> --- a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>> +++ b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>> @@ -296,6 +296,7 @@ static int visionox_vtdr6130_probe(struct 
>> mipi_dsi_device *dsi)
>>       dsi->format = MIPI_DSI_FMT_RGB888;
>>       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | 
>> MIPI_DSI_MODE_NO_EOT_PACKET |
>>                 MIPI_DSI_CLOCK_NON_CONTINUOUS;
>> +    ctx->panel.prepare_prev_first = true;
>>       drm_panel_init(&ctx->panel, dev, &visionox_vtdr6130_panel_funcs,
>>                  DRM_MODE_CONNECTOR_DSI);
>>
>> ---
>> base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
>> change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f
>>
>> Best regards,
>
  

Hi Neil

On 8/3/2023 10:19 AM, Jessica Zhang wrote:
> 
> 
> On 7/31/2023 6:00 AM, Neil Armstrong wrote:
>> Hi,
>>
>> On 26/07/2023 00:56, Jessica Zhang wrote:
>>> Due to a recent introduction of the pre_enable_prev_first bridge flag 
>>> [1],
>>> the panel driver will be probed before the DSI is enabled, causing the
>>> DCS commands to fail to send.
>>>
>>> Ensure that DSI is enabled before panel probe by setting the
>>> prepare_prev_first flag for the panel.
>>
>> Well this is specific to MSM DSI driver, it's not related at all to 
>> the panel.
> 

I dont fully agree this is a MSM DSI driver specific thing.

If the panel can send its commands in its enable() callback, then this 
flag need not be set.

When a panel sends its DCS commands in its pre_enable() callback, any 
DSI controller will need to be ON before that otherwise DCS commands 
cannot be sent.

With this in mind, may I know why is this a MSM change and not a panel 
change?

As per my discussion with Dmitry during the last sync up, we were 
aligned on this expectation.

Thanks

Abhinav

> Hi Neil,
> 
> I think there might be some confusion caused by the commit message -- 
> instead of "enabled before panel probe", it should be "enabled before 
> panel pre_enable()" as the panel on commands are sent during prepare(), 
> which is matched to bridge pre_enable().
> 
> IIRC the general rule is that the panel driver should set the 
> prepare_prev_first flag if the on commands are sent during pre_enable(), 
> so I'll keep the code change but correct the commit message if that's ok 
> with you.
> 
> Thanks,
> 

> Jessica Zhang
> 
>>
>> Neil
>>
>>>
>>> [1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first 
>>> to alter bridge init order")
>>>
>>> Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel 
>>> driver")
>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>> ---
>>>   drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c 
>>> b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>> index bb0dfd86ea67..e1363e128e7e 100644
>>> --- a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>> +++ b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>> @@ -296,6 +296,7 @@ static int visionox_vtdr6130_probe(struct 
>>> mipi_dsi_device *dsi)
>>>       dsi->format = MIPI_DSI_FMT_RGB888;
>>>       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | 
>>> MIPI_DSI_MODE_NO_EOT_PACKET |
>>>                 MIPI_DSI_CLOCK_NON_CONTINUOUS;
>>> +    ctx->panel.prepare_prev_first = true;
>>>       drm_panel_init(&ctx->panel, dev, &visionox_vtdr6130_panel_funcs,
>>>                  DRM_MODE_CONNECTOR_DSI);
>>>
>>> ---
>>> base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
>>> change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f
>>>
>>> Best regards,
>>
  

Hi Abhinav,

On 10/08/2023 18:26, Abhinav Kumar wrote:
> Hi Neil
> 
> On 8/3/2023 10:19 AM, Jessica Zhang wrote:
>>
>>
>> On 7/31/2023 6:00 AM, Neil Armstrong wrote:
>>> Hi,
>>>
>>> On 26/07/2023 00:56, Jessica Zhang wrote:
>>>> Due to a recent introduction of the pre_enable_prev_first bridge flag [1],
>>>> the panel driver will be probed before the DSI is enabled, causing the
>>>> DCS commands to fail to send.
>>>>
>>>> Ensure that DSI is enabled before panel probe by setting the
>>>> prepare_prev_first flag for the panel.
>>>
>>> Well this is specific to MSM DSI driver, it's not related at all to the panel.
>>
> 
> I dont fully agree this is a MSM DSI driver specific thing.
> 
> If the panel can send its commands in its enable() callback, then this flag need not be set.
> 
> When a panel sends its DCS commands in its pre_enable() callback, any DSI controller will need to be ON before that otherwise DCS commands cannot be sent.
> 
> With this in mind, may I know why is this a MSM change and not a panel change?
> 
> As per my discussion with Dmitry during the last sync up, we were aligned on this expectation.

As of today, only the MSM DSI driver expects panels to have prepare_prev_first because it's the first
one calling pre_enable() before the DSI controller to be on, all other DSI drivers I know
still enables the DSI controller in mode_set() and thus can send commands in pre_enable() which
is a loose way to map the pre-video state for DSI panels...

A panel driver should not depend on features of a DSI controller, which is the case here
with this patch. Today's expectation is to send DSI commands in pre_enable() then when enabled
expect to be in video mode when enable() is called.

The main reason is because some DSI controllers cannot send LP commands after switching
to video mode (allwinner for example), so we must take this into account.

For v6.6, I don't see other solutions than reverting 9e15123eca79 (reverting won't regress anything,
because now it regresses also other panels on MSM platforms) and try to find a proper solution for v6.7...

Neil

> 
> Thanks
> 
> Abhinav
> 
>> Hi Neil,
>>
>> I think there might be some confusion caused by the commit message -- instead of "enabled before panel probe", it should be "enabled before panel pre_enable()" as the panel on commands are sent during prepare(), which is matched to bridge pre_enable().
>>
>> IIRC the general rule is that the panel driver should set the prepare_prev_first flag if the on commands are sent during pre_enable(), so I'll keep the code change but correct the commit message if that's ok with you.
>>
>> Thanks,
>>
> 
>> Jessica Zhang
>>
>>>
>>> Neil
>>>
>>>>
>>>> [1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")

It's not the right commit that cause regression here, it's :

9e15123eca79 drm/msm/dsi: Stop unconditionally powering up DSI hosts at modeset

>>>>
>>>> Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel driver")
>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>>> ---
>>>>   drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
>>>>   1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>> index bb0dfd86ea67..e1363e128e7e 100644
>>>> --- a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>> +++ b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>> @@ -296,6 +296,7 @@ static int visionox_vtdr6130_probe(struct mipi_dsi_device *dsi)
>>>>       dsi->format = MIPI_DSI_FMT_RGB888;
>>>>       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_NO_EOT_PACKET |
>>>>                 MIPI_DSI_CLOCK_NON_CONTINUOUS;
>>>> +    ctx->panel.prepare_prev_first = true;
>>>>       drm_panel_init(&ctx->panel, dev, &visionox_vtdr6130_panel_funcs,
>>>>                  DRM_MODE_CONNECTOR_DSI);
>>>>
>>>> ---
>>>> base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
>>>> change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f
>>>>
>>>> Best regards,
>>>
  

Hi Abhinav,

On 14/08/2023 20:02, Abhinav Kumar wrote:
> Hi Neil
> 
> On 8/14/2023 1:01 AM, neil.armstrong@linaro.org wrote:
>> Hi Abhinav,
>>
>> On 10/08/2023 18:26, Abhinav Kumar wrote:
>>> Hi Neil
>>>
>>> On 8/3/2023 10:19 AM, Jessica Zhang wrote:
>>>>
>>>>
>>>> On 7/31/2023 6:00 AM, Neil Armstrong wrote:
>>>>> Hi,
>>>>>
>>>>> On 26/07/2023 00:56, Jessica Zhang wrote:
>>>>>> Due to a recent introduction of the pre_enable_prev_first bridge flag [1],
>>>>>> the panel driver will be probed before the DSI is enabled, causing the
>>>>>> DCS commands to fail to send.
>>>>>>
>>>>>> Ensure that DSI is enabled before panel probe by setting the
>>>>>> prepare_prev_first flag for the panel.
>>>>>
>>>>> Well this is specific to MSM DSI driver, it's not related at all to the panel.
>>>>
>>>
>>> I dont fully agree this is a MSM DSI driver specific thing.
>>>
>>> If the panel can send its commands in its enable() callback, then this flag need not be set.
>>>
>>> When a panel sends its DCS commands in its pre_enable() callback, any DSI controller will need to be ON before that otherwise DCS commands cannot be sent.
>>>
>>> With this in mind, may I know why is this a MSM change and not a panel change?
>>>
>>> As per my discussion with Dmitry during the last sync up, we were aligned on this expectation.
>>
>> As of today, only the MSM DSI driver expects panels to have prepare_prev_first because it's the first
>> one calling pre_enable() before the DSI controller to be on, all other DSI drivers I know
>> still enables the DSI controller in mode_set() and thus can send commands in pre_enable() which
>> is a loose way to map the pre-video state for DSI panels...
>>
> 
> It looks like there are multiple panels already setting this flag so this panel will not be the first unless they were added to make those work with MSM (which seems unlikely)
> 
> panel-samsung-s6d7aa0.c:        ctx->panel.prepare_prev_first = true;
> panel-samsung-s6e3ha2.c:        ctx->panel.prepare_prev_first = true;
> panel-samsung-s6e63j0x03.c:     ctx->panel.prepare_prev_first = true;
> panel-samsung-s6e8aa0.c:        ctx->panel.prepare_prev_first = true;
> 
> This is where I would like to understand a bit that if the panel sends out the ON commands in enable() instead of pre_enable() then, this flag will not be needed. So its also depends on the panel side and thats why
> the bridge feeds of the panel's input in devm_drm_panel_bridge_add_typed()
> 
> bridge->pre_enable_prev_first = panel->prepare_prev_first;
> 
>> A panel driver should not depend on features of a DSI controller, which is the case here
>> with this patch. Today's expectation is to send DSI commands in pre_enable() then when enabled
>> expect to be in video mode when enable() is called.
>>
> 
> We are not depending on any feature as such. Any DSI controller , not just MSM's would need to be ON for DCS commands to be sent out in the panel's pre_enable() callback.
> 
> Its not true that MSM is the only driver powering on the DSI controller in pre_enable(). Even MTK seems to be doing that
> 
> mtk_dsi_bridge_atomic_pre_enable
> 
> So I assume any panel which sends out commands in pre_enable() will not work with MTK as well.

Sending HS commands will always work on any controller, it's all about LP commands.
The Samsung panels you listed only send HS commands so they can use prepare_prev_first
and work on any controllers.

None of the panels using LP commands uses prepare_prev_first:

$ grep prepare_prev_first `grep -l LPM drivers/gpu/drm/panel/panel-*`
$

Note that there's a smart move for VTDR6130 with the command mode introduced in 20230728011218.14630-1-parellan@quicinc.com,
in the way prepare_prev_first could only be set to true if command-mode is selected.
I'll accept that since it would be logical, video mode won't work anymore but by default
the panel would still work in command mode + prepare_prev_first.

> 
>> The main reason is because some DSI controllers cannot send LP commands after switching
>> to video mode (allwinner for example), so we must take this into account.
>>
>> For v6.6, I don't see other solutions than reverting 9e15123eca79 (reverting won't regress anything,
>> because now it regresses also other panels on MSM platforms) and try to find a proper solution for v6.7...
>>
> 
> No, I would prefer not to revert that. It will bring back special handling for the parade chip into MSM driver, something which I would prefer not to go back to. Powering on the DSI in modeset() was done only for the parade chip.

I understand, but this patch doesn't qualify as a fix for 9e15123eca79 and is too late to be merged in drm-misc-next for v6.6,
and since 9e15123eca79 actually breaks some support it should be reverted (+ deps) since we are late in the rc cycles.

It's not a fatality or the end of the world, but this is an indirect fix and not way all this should be fixed.
We already had the case for the lt9611 breakage, and it's the same case here.

Neil

> 
>> Neil
>>
>>>
>>> Thanks
>>>
>>> Abhinav
>>>
>>>> Hi Neil,
>>>>
>>>> I think there might be some confusion caused by the commit message -- instead of "enabled before panel probe", it should be "enabled before panel pre_enable()" as the panel on commands are sent during prepare(), which is matched to bridge pre_enable().
>>>>
>>>> IIRC the general rule is that the panel driver should set the prepare_prev_first flag if the on commands are sent during pre_enable(), so I'll keep the code change but correct the commit message if that's ok with you.
>>>>
>>>> Thanks,
>>>>
>>>
>>>> Jessica Zhang
>>>>
>>>>>
>>>>> Neil
>>>>>
>>>>>>
>>>>>> [1] commit 4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order")
>>
>> It's not the right commit that cause regression here, it's :
>>
>> 9e15123eca79 drm/msm/dsi: Stop unconditionally powering up DSI hosts at modeset
>>
>>>>>>
>>>>>> Fixes: 2349183d32d8 ("drm/panel: add visionox vtdr6130 DSI panel driver")
>>>>>> Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
>>>>>> ---
>>>>>>   drivers/gpu/drm/panel/panel-visionox-vtdr6130.c | 1 +
>>>>>>   1 file changed, 1 insertion(+)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>>>> index bb0dfd86ea67..e1363e128e7e 100644
>>>>>> --- a/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>>>> +++ b/drivers/gpu/drm/panel/panel-visionox-vtdr6130.c
>>>>>> @@ -296,6 +296,7 @@ static int visionox_vtdr6130_probe(struct mipi_dsi_device *dsi)
>>>>>>       dsi->format = MIPI_DSI_FMT_RGB888;
>>>>>>       dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_NO_EOT_PACKET |
>>>>>>                 MIPI_DSI_CLOCK_NON_CONTINUOUS;
>>>>>> +    ctx->panel.prepare_prev_first = true;
>>>>>>       drm_panel_init(&ctx->panel, dev, &visionox_vtdr6130_panel_funcs,
>>>>>>                  DRM_MODE_CONNECTOR_DSI);
>>>>>>
>>>>>> ---
>>>>>> base-commit: 28a5c036b05fc5c935cc72d76abd3589825ea9cd
>>>>>> change-id: 20230717-visionox-vtdr-prev-first-e00ae02eec9f
>>>>>>
>>>>>> Best regards,
>>>>>
>>
  

On 18/08/2023 11:25, neil.armstrong@linaro.org wrote:
> Hi Dmitry,
> 
> On 17/08/2023 20:35, Dmitry Baryshkov wrote:
>> On 16/08/2023 10:51, neil.armstrong@linaro.org wrote:
>>> Hi Abhinav,
>>>
>>> On 14/08/2023 20:02, Abhinav Kumar wrote:
> 
> <snip>
> 
>>>
>>> Sending HS commands will always work on any controller, it's all 
>>> about LP commands.
>>> The Samsung panels you listed only send HS commands so they can use 
>>> prepare_prev_first
>>> and work on any controllers.
>>
>> I think there is some misunderstanding there, supported by the 
>> description of the flag.
>>
>> If I remember correctly, some hosts (sunxi) can not send DCS commands 
>> after enabling video stream and switching to HS mode, see [1]. Thus, 
>> as you know, most of the drivers have all DSI panel setup commands in 
>> drm_panel_funcs::prepare() / drm_bridge_funcs::pre_enable() callbacks, 
>> not paying attention whether these commands are to be sent in LP or in 
>> HS mode.
>>
>> Previously DSI source drivers could power on the DSI link either in 
>> mode_set() or in pre_enable() callbacks, with mode_set() being the 
>> hack to make panel/bridge drivers to be able to send commands from 
>> their prepare() / pre_enable() callbacks.
>>
>> With the prev_first flags being introduced, we have established that 
>> DSI link should be enabled in DSI host's pre_enable() callback and 
>> switched to HS mode (be it command or video) in the enable() callback.
>>
>> So far so good.
> 
> It seems coherent, I would like first to have a state of all DSI host 
> drivers and make this would actually work first before adding the 
> prev_first flag to all the required panels.
> 
>>
>> Unfortunately this change is not fully backwards-compatible. This 
>> requires that all DSI panels sending commands from prepare() should 
>> have the prepare_prev_first flag. In some sense, all such patches 
>> might have Fixes: 5ea6b1702781 ("drm/panel: Add prepare_prev_first 
>> flag to drm_panel").
> 
> This kind of migration should be done *before* any possible regression, 
> not the other way round.
> 
> If all panels sending commands from prepare() should have the 
> prepare_prev_first flag, then it should be first, check for regressions 
> then continue.
> 
> <snip>
> 
>>>
>>> I understand, but this patch doesn't qualify as a fix for 
>>> 9e15123eca79 and is too late to be merged in drm-misc-next for v6.6,
>>> and since 9e15123eca79 actually breaks some support it should be 
>>> reverted (+ deps) since we are late in the rc cycles.
>>
>> If we go this way, we can never reapply these patches. There will be 
>> no guarantee that all panel drivers are completely converted. We 
>> already have a story without an observable end - 
>> DRM_BRIDGE_ATTACH_NO_CONNECTOR.
> 
> I don't understand this point, who would block re-applying the patches ?

Consider us reverting 9e15123eca79 now and then reapplying it next 
cycle. Then another panel / bridge that was not converted to use 
pre_enable_prev_first pops up. And suddently we have to revert them again.

> The migration to DRM_BRIDGE_ATTACH_NO_CONNECTOR was done over multiple 
> Linux version and went smoothly because we reverted
> regressing patches and restarted when needed, I don't understand why we 
> can't do this here aswell.

With DRM_BRIDGE_ATTACH_NO_CONNECTOR both host and peripheral drivers 
were involved. This way they share knowledge about the migration state.

With prev_first we do not have such shared knowledge. Host assumes that 
it can work according to the documentation: turn DSI link to LP-11 in 
pre_enable(), switch to HS in enable(). It can not check whether the 
next bridge did not set pre_enable_prev_first because of it not being 
required (like for the Parade bridge) or because next bridge is not 
converted yet (and thus DSI host should power up the link in 
atomic_mode_set).

Granted that there is no way for the DSI host driver to attune itself to 
the DSI peripheral driver requirements, I can only consider 
corresponding (requiring prev_first) panel drivers broken since 
5ea6b1702781 ("drm/panel: Add prepare_prev_first flag to drm_panel") and 
all bridge drivers with this issue broken since 4fb912e5e190 
("drm/bridge: Introduce pre_enable_prev_first to alter bridge init order").

> 
>>
>> I'd consider that the DSI driver is correct here and it is about the 
>> panel drivers that require fixes patches. If you care about the 
>> particular Fixes tag, I have provided one several lines above.
> 
> Unfortunately it should be done in the other way round, prepare for 
> migration, then migrate,
> 
> I mean if it's a required migration, then it should be done and I'll 
> support it from both bridge and panel PoV.
> 
> So, first this patch has the wrong Fixes tag, and I would like a better 
> explanation on the commit message in any case.
> Then I would like to have an ack from some drm-misc maintainers before 
> applying it because it fixes a patch that
> was sent via the msm tree thus per the drm-misc rules I cannot apply it 
> via the drm-misc-next-fixes tree.
> 
> Neil
> 
> <snip>
>
  

Hi,

On Fri, Aug 18, 2023 at 10:25:48AM +0200, neil.armstrong@linaro.org wrote:
> On 17/08/2023 20:35, Dmitry Baryshkov wrote:
> > On 16/08/2023 10:51, neil.armstrong@linaro.org wrote:
> > > Sending HS commands will always work on any controller, it's all
> > > about LP commands. The Samsung panels you listed only send HS
> > > commands so they can use prepare_prev_first and work on any
> > > controllers.
> > 
> > I think there is some misunderstanding there, supported by the
> > description of the flag.
> > 
> > If I remember correctly, some hosts (sunxi) can not send DCS
> > commands after enabling video stream and switching to HS mode, see
> > [1]. Thus, as you know, most of the drivers have all DSI panel setup
> > commands in drm_panel_funcs::prepare() /
> > drm_bridge_funcs::pre_enable() callbacks, not paying attention
> > whether these commands are to be sent in LP or in HS mode.
> > 
> > Previously DSI source drivers could power on the DSI link either in
> > mode_set() or in pre_enable() callbacks, with mode_set() being the
> > hack to make panel/bridge drivers to be able to send commands from
> > their prepare() / pre_enable() callbacks.
> > 
> > With the prev_first flags being introduced, we have established that
> > DSI link should be enabled in DSI host's pre_enable() callback and
> > switched to HS mode (be it command or video) in the enable()
> > callback.
> >
> > So far so good.
> 
> It seems coherent, I would like first to have a state of all DSI host
> drivers and make this would actually work first before adding the
> prev_first flag to all the required panels.

This is definitely what we should do in an ideal world, but at least for
sunxi there's no easy way for it at the moment. There's no documentation
for it and the driver provided doesn't allow this to happen.

Note that I'm not trying to discourage you or something here, I'm simply
pointing out that this will be something that we will have to take into
account. And it's possible that other drivers are in a similar
situation.

> > Unfortunately this change is not fully backwards-compatible. This
> > requires that all DSI panels sending commands from prepare() should
> > have the prepare_prev_first flag. In some sense, all such patches
> > might have Fixes: 5ea6b1702781 ("drm/panel: Add prepare_prev_first
> > flag to drm_panel").
> 
> This kind of migration should be done *before* any possible
> regression, not the other way round.
> 
> If all panels sending commands from prepare() should have the
> prepare_prev_first flag, then it should be first, check for
> regressions then continue.
> 
> <snip>
> 
> > > 
> > > I understand, but this patch doesn't qualify as a fix for
> > > 9e15123eca79 and is too late to be merged in drm-misc-next for
> > > v6.6, and since 9e15123eca79 actually breaks some support it
> > > should be reverted (+ deps) since we are late in the rc cycles.
> > 
> > If we go this way, we can never reapply these patches. There will be
> > no guarantee that all panel drivers are completely converted. We
> > already have a story without an observable end -
> > DRM_BRIDGE_ATTACH_NO_CONNECTOR.
> 
> I don't understand this point, who would block re-applying the patches ?
> 
> The migration to DRM_BRIDGE_ATTACH_NO_CONNECTOR was done over multiple
> Linux version and went smoothly because we reverted regressing patches
> and restarted when needed, I don't understand why we can't do this
> here aswell.
> 
> > I'd consider that the DSI driver is correct here and it is about the
> > panel drivers that require fixes patches. If you care about the
> > particular Fixes tag, I have provided one several lines above.
> 
> Unfortunately it should be done in the other way round, prepare for
> migration, then migrate,
> 
> I mean if it's a required migration, then it should be done and I'll
> support it from both bridge and panel PoV.
> 
> So, first this patch has the wrong Fixes tag, and I would like a
> better explanation on the commit message in any case. Then I would
> like to have an ack from some drm-misc maintainers before applying it
> because it fixes a patch that was sent via the msm tree thus per the
> drm-misc rules I cannot apply it via the drm-misc-next-fixes tree.

Sorry, it's not clear to me what you'd like our feedback on exactly?

Maxime