diff mbox series

[2/2] pwm: mtk-disp: Configure double buffering before reading in .get_state()

Message ID 20230123160615.375969-3-angelogioacchino.delregno@collabora.com
State New
Headers
Series pwm: mtk-disp: Fix backlight configuration at boot |

Commit Message

AngeloGioacchino Del Regno Jan. 23, 2023, 4:06 p.m. UTC 
  The DISP_PWM controller's default behavior is to always use register
double buffering: all reads/writes are then performed on shadow
registers instead of working registers and this becomes an issue
in case our chosen configuration in Linux is different from the
default (or from the one that was pre-applied by the bootloader).

An example of broken behavior is when the controller is configured
to use shadow registers, but this driver wants to configure it
otherwise: what happens is that the .get_state() callback is called
right after registering the pwmchip and checks whether the PWM is
enabled by reading the DISP_PWM_EN register;
At this point, if shadow registers are enabled but their content
was not committed before booting Linux, we are *not* reading the
current PWM enablement status, leading to the kernel knowing that
the hardware is actually enabled when, in reality, it's not.

The aforementioned issue emerged since this driver was fixed with
commit 0b5ef3429d8f ("pwm: mtk-disp: Fix the parameters calculated
by the enabled flag of disp_pwm") making it to read the enablement
status from the right register.

Configure the controller in the .get_state() callback to avoid
this desync issue and get the backlight properly working again.

Fixes: 3f2b16734914 ("pwm: mtk-disp: Implement atomic API .get_state()")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
---
 drivers/pwm/pwm-mtk-disp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

Comments

Nícolas F. R. A. Prado Jan. 26, 2023, 3:19 p.m. UTC | #1 
On Mon, Jan 23, 2023 at 05:06:15PM +0100, AngeloGioacchino Del Regno wrote:
> The DISP_PWM controller's default behavior is to always use register
> double buffering: all reads/writes are then performed on shadow
> registers instead of working registers and this becomes an issue
> in case our chosen configuration in Linux is different from the
> default (or from the one that was pre-applied by the bootloader).
> 
> An example of broken behavior is when the controller is configured
> to use shadow registers, but this driver wants to configure it
> otherwise: what happens is that the .get_state() callback is called
> right after registering the pwmchip and checks whether the PWM is
> enabled by reading the DISP_PWM_EN register;
> At this point, if shadow registers are enabled but their content
> was not committed before booting Linux, we are *not* reading the
> current PWM enablement status, leading to the kernel knowing that
> the hardware is actually enabled when, in reality, it's not.
> 
> The aforementioned issue emerged since this driver was fixed with
> commit 0b5ef3429d8f ("pwm: mtk-disp: Fix the parameters calculated
> by the enabled flag of disp_pwm") making it to read the enablement
> status from the right register.
> 
> Configure the controller in the .get_state() callback to avoid
> this desync issue and get the backlight properly working again.
> 
> Fixes: 3f2b16734914 ("pwm: mtk-disp: Implement atomic API .get_state()")
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> ---
>  drivers/pwm/pwm-mtk-disp.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-mtk-disp.c b/drivers/pwm/pwm-mtk-disp.c
> index 82b430d881a2..fe9593f968ee 100644
> --- a/drivers/pwm/pwm-mtk-disp.c
> +++ b/drivers/pwm/pwm-mtk-disp.c
> @@ -196,6 +196,16 @@ static int mtk_disp_pwm_get_state(struct pwm_chip *chip,
>  		return err;
>  	}
>  
> +	/*
> +	 * Apply DISP_PWM_DEBUG settings to choose whether to enable or disable
> +	 * registers double buffer and manual commit to working register before
> +	 * performing any read/write operation
> +	 */
> +	if (mdp->data->bls_debug)

I feel like this condition should be the same as in the apply() callback, since
they're doing the same write operation, so also have '&& !has_commit'.

Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>

On MT8192 Asurada Spherion.

Thanks,
Nícolas

> +		mtk_disp_pwm_update_bits(mdp, mdp->data->bls_debug,
> +					 mdp->data->bls_debug_mask,
> +					 mdp->data->bls_debug_mask);
> +
>  	rate = clk_get_rate(mdp->clk_main);
>  	con0 = readl(mdp->base + mdp->data->con0);
>  	con1 = readl(mdp->base + mdp->data->con1);
> -- 
> 2.39.0
> 
>
AngeloGioacchino Del Regno Jan. 26, 2023, 3:24 p.m. UTC | #2 
Il 26/01/23 16:19, Nícolas F. R. A. Prado ha scritto:
> On Mon, Jan 23, 2023 at 05:06:15PM +0100, AngeloGioacchino Del Regno wrote:
>> The DISP_PWM controller's default behavior is to always use register
>> double buffering: all reads/writes are then performed on shadow
>> registers instead of working registers and this becomes an issue
>> in case our chosen configuration in Linux is different from the
>> default (or from the one that was pre-applied by the bootloader).
>>
>> An example of broken behavior is when the controller is configured
>> to use shadow registers, but this driver wants to configure it
>> otherwise: what happens is that the .get_state() callback is called
>> right after registering the pwmchip and checks whether the PWM is
>> enabled by reading the DISP_PWM_EN register;
>> At this point, if shadow registers are enabled but their content
>> was not committed before booting Linux, we are *not* reading the
>> current PWM enablement status, leading to the kernel knowing that
>> the hardware is actually enabled when, in reality, it's not.
>>
>> The aforementioned issue emerged since this driver was fixed with
>> commit 0b5ef3429d8f ("pwm: mtk-disp: Fix the parameters calculated
>> by the enabled flag of disp_pwm") making it to read the enablement
>> status from the right register.
>>
>> Configure the controller in the .get_state() callback to avoid
>> this desync issue and get the backlight properly working again.
>>
>> Fixes: 3f2b16734914 ("pwm: mtk-disp: Implement atomic API .get_state()")
>> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
>> ---
>>   drivers/pwm/pwm-mtk-disp.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/drivers/pwm/pwm-mtk-disp.c b/drivers/pwm/pwm-mtk-disp.c
>> index 82b430d881a2..fe9593f968ee 100644
>> --- a/drivers/pwm/pwm-mtk-disp.c
>> +++ b/drivers/pwm/pwm-mtk-disp.c
>> @@ -196,6 +196,16 @@ static int mtk_disp_pwm_get_state(struct pwm_chip *chip,
>>   		return err;
>>   	}
>>   
>> +	/*
>> +	 * Apply DISP_PWM_DEBUG settings to choose whether to enable or disable
>> +	 * registers double buffer and manual commit to working register before
>> +	 * performing any read/write operation
>> +	 */
>> +	if (mdp->data->bls_debug)
> 
> I feel like this condition should be the same as in the apply() callback, since
> they're doing the same write operation, so also have '&& !has_commit'.
> 

The bls_debug register is used to both enable and/or disable various features,
including the one that I'm targeting in this commit, which is disabling shadow
registers.

As I explained in the commit message, we don't want to - and cannot - assume that
the bootloader doesn't *reset* the backlight controller before booting Linux: a
reset would re-enable the shadow registers, and this function being called as
first to check the backlight EN status may fail to do so.

This is as well true in the opposite situation where, in the future, we may want
to set shadow registers ON, while the bootloader sets them OFF before booting:
adding a (x && !has_commit) check in this branch would defeat that purpose and
make this commit... well.. partially broken! :-)

Cheers!
Angelo

> Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> 
> On MT8192 Asurada Spherion.
> 
> Thanks,
> Nícolas
> 
>> +		mtk_disp_pwm_update_bits(mdp, mdp->data->bls_debug,
>> +					 mdp->data->bls_debug_mask,
>> +					 mdp->data->bls_debug_mask);
>> +
>>   	rate = clk_get_rate(mdp->clk_main);
>>   	con0 = readl(mdp->base + mdp->data->con0);
>>   	con1 = readl(mdp->base + mdp->data->con1);
>> -- 
>> 2.39.0
>>
>>
Nícolas F. R. A. Prado Jan. 26, 2023, 4:09 p.m. UTC | #3 
On Thu, Jan 26, 2023 at 04:24:29PM +0100, AngeloGioacchino Del Regno wrote:
> Il 26/01/23 16:19, Nícolas F. R. A. Prado ha scritto:
> > On Mon, Jan 23, 2023 at 05:06:15PM +0100, AngeloGioacchino Del Regno wrote:
> > > The DISP_PWM controller's default behavior is to always use register
> > > double buffering: all reads/writes are then performed on shadow
> > > registers instead of working registers and this becomes an issue
> > > in case our chosen configuration in Linux is different from the
> > > default (or from the one that was pre-applied by the bootloader).
> > > 
> > > An example of broken behavior is when the controller is configured
> > > to use shadow registers, but this driver wants to configure it
> > > otherwise: what happens is that the .get_state() callback is called
> > > right after registering the pwmchip and checks whether the PWM is
> > > enabled by reading the DISP_PWM_EN register;
> > > At this point, if shadow registers are enabled but their content
> > > was not committed before booting Linux, we are *not* reading the
> > > current PWM enablement status, leading to the kernel knowing that
> > > the hardware is actually enabled when, in reality, it's not.
> > > 
> > > The aforementioned issue emerged since this driver was fixed with
> > > commit 0b5ef3429d8f ("pwm: mtk-disp: Fix the parameters calculated
> > > by the enabled flag of disp_pwm") making it to read the enablement
> > > status from the right register.
> > > 
> > > Configure the controller in the .get_state() callback to avoid
> > > this desync issue and get the backlight properly working again.
> > > 
> > > Fixes: 3f2b16734914 ("pwm: mtk-disp: Implement atomic API .get_state()")
> > > Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> > > ---
> > >   drivers/pwm/pwm-mtk-disp.c | 10 ++++++++++
> > >   1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/drivers/pwm/pwm-mtk-disp.c b/drivers/pwm/pwm-mtk-disp.c
> > > index 82b430d881a2..fe9593f968ee 100644
> > > --- a/drivers/pwm/pwm-mtk-disp.c
> > > +++ b/drivers/pwm/pwm-mtk-disp.c
> > > @@ -196,6 +196,16 @@ static int mtk_disp_pwm_get_state(struct pwm_chip *chip,
> > >   		return err;
> > >   	}
> > > +	/*
> > > +	 * Apply DISP_PWM_DEBUG settings to choose whether to enable or disable
> > > +	 * registers double buffer and manual commit to working register before
> > > +	 * performing any read/write operation
> > > +	 */
> > > +	if (mdp->data->bls_debug)
> > 
> > I feel like this condition should be the same as in the apply() callback, since
> > they're doing the same write operation, so also have '&& !has_commit'.
> > 
> 
> The bls_debug register is used to both enable and/or disable various features,
> including the one that I'm targeting in this commit, which is disabling shadow
> registers.
> 
> As I explained in the commit message, we don't want to - and cannot - assume that
> the bootloader doesn't *reset* the backlight controller before booting Linux: a
> reset would re-enable the shadow registers, and this function being called as
> first to check the backlight EN status may fail to do so.
> 
> This is as well true in the opposite situation where, in the future, we may want
> to set shadow registers ON, while the bootloader sets them OFF before booting:
> adding a (x && !has_commit) check in this branch would defeat that purpose and
> make this commit... well.. partially broken! :-)

Makes sense, but in that case shouldn't we drop the (&& !has_commit) in the
check of the previous commit too? I get that in the pwm's core current logic,
get_state() is run before apply(), but given that we also write the debug
register in apply(), we're not relying on that. So as it currently stands, if in
the future the bootloader sets shadow registers OFF, and we want to set them ON,
and we call apply() before having called get_state(), we'd be back to the broken
behavior.

Thanks,
Nícolas

> 
> Cheers!
> Angelo
> 
> > Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> > Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> > 
> > On MT8192 Asurada Spherion.
> > 
> > Thanks,
> > Nícolas
> > 
> > > +		mtk_disp_pwm_update_bits(mdp, mdp->data->bls_debug,
> > > +					 mdp->data->bls_debug_mask,
> > > +					 mdp->data->bls_debug_mask);
> > > +
> > >   	rate = clk_get_rate(mdp->clk_main);
> > >   	con0 = readl(mdp->base + mdp->data->con0);
> > >   	con1 = readl(mdp->base + mdp->data->con1);
> > > -- 
> > > 2.39.0
> > > 
> > > 
> 
>
AngeloGioacchino Del Regno Jan. 26, 2023, 4:16 p.m. UTC | #4 
Il 26/01/23 17:09, Nícolas F. R. A. Prado ha scritto:
> On Thu, Jan 26, 2023 at 04:24:29PM +0100, AngeloGioacchino Del Regno wrote:
>> Il 26/01/23 16:19, Nícolas F. R. A. Prado ha scritto:
>>> On Mon, Jan 23, 2023 at 05:06:15PM +0100, AngeloGioacchino Del Regno wrote:
>>>> The DISP_PWM controller's default behavior is to always use register
>>>> double buffering: all reads/writes are then performed on shadow
>>>> registers instead of working registers and this becomes an issue
>>>> in case our chosen configuration in Linux is different from the
>>>> default (or from the one that was pre-applied by the bootloader).
>>>>
>>>> An example of broken behavior is when the controller is configured
>>>> to use shadow registers, but this driver wants to configure it
>>>> otherwise: what happens is that the .get_state() callback is called
>>>> right after registering the pwmchip and checks whether the PWM is
>>>> enabled by reading the DISP_PWM_EN register;
>>>> At this point, if shadow registers are enabled but their content
>>>> was not committed before booting Linux, we are *not* reading the
>>>> current PWM enablement status, leading to the kernel knowing that
>>>> the hardware is actually enabled when, in reality, it's not.
>>>>
>>>> The aforementioned issue emerged since this driver was fixed with
>>>> commit 0b5ef3429d8f ("pwm: mtk-disp: Fix the parameters calculated
>>>> by the enabled flag of disp_pwm") making it to read the enablement
>>>> status from the right register.
>>>>
>>>> Configure the controller in the .get_state() callback to avoid
>>>> this desync issue and get the backlight properly working again.
>>>>
>>>> Fixes: 3f2b16734914 ("pwm: mtk-disp: Implement atomic API .get_state()")
>>>> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
>>>> ---
>>>>    drivers/pwm/pwm-mtk-disp.c | 10 ++++++++++
>>>>    1 file changed, 10 insertions(+)
>>>>
>>>> diff --git a/drivers/pwm/pwm-mtk-disp.c b/drivers/pwm/pwm-mtk-disp.c
>>>> index 82b430d881a2..fe9593f968ee 100644
>>>> --- a/drivers/pwm/pwm-mtk-disp.c
>>>> +++ b/drivers/pwm/pwm-mtk-disp.c
>>>> @@ -196,6 +196,16 @@ static int mtk_disp_pwm_get_state(struct pwm_chip *chip,
>>>>    		return err;
>>>>    	}
>>>> +	/*
>>>> +	 * Apply DISP_PWM_DEBUG settings to choose whether to enable or disable
>>>> +	 * registers double buffer and manual commit to working register before
>>>> +	 * performing any read/write operation
>>>> +	 */
>>>> +	if (mdp->data->bls_debug)
>>>
>>> I feel like this condition should be the same as in the apply() callback, since
>>> they're doing the same write operation, so also have '&& !has_commit'.
>>>
>>
>> The bls_debug register is used to both enable and/or disable various features,
>> including the one that I'm targeting in this commit, which is disabling shadow
>> registers.
>>
>> As I explained in the commit message, we don't want to - and cannot - assume that
>> the bootloader doesn't *reset* the backlight controller before booting Linux: a
>> reset would re-enable the shadow registers, and this function being called as
>> first to check the backlight EN status may fail to do so.
>>
>> This is as well true in the opposite situation where, in the future, we may want
>> to set shadow registers ON, while the bootloader sets them OFF before booting:
>> adding a (x && !has_commit) check in this branch would defeat that purpose and
>> make this commit... well.. partially broken! :-)
> 
> Makes sense, but in that case shouldn't we drop the (&& !has_commit) in the
> check of the previous commit too? I get that in the pwm's core current logic,

No. The previous commit checks !has_commit to select a register write strategy
between "shadow -> commit" and "working registers - no commit necessary".

If you drop that check from the previous commit, how can you choose the write
strategy to use?! :-)

You can't rely on reading the bls_debug register, either, because it may be
holding different values compared to what we want due to, for example, a reset
and you can't rely on checking bls_debug_mask because in the future this kind
of selector may be residing in an entirely different register.

> get_state() is run before apply(), but given that we also write the debug
> register in apply(), we're not relying on that. So as it currently stands, if in
> the future the bootloader sets shadow registers OFF, and we want to set them ON,
> and we call apply() before having called get_state(), we'd be back to the broken
> behavior.

No, because the default BLS_DEBUG register value at IP reset is 0 - where 0 means
"do not disable commit, do not disable shadow register read, do not disable shadow
register write" :-)

Cheers,
Angelo

> 
> Thanks,
> Nícolas
> 
>>
>> Cheers!
>> Angelo
>>
>>> Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
>>> Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
>>>
>>> On MT8192 Asurada Spherion.
>>>
>>> Thanks,
>>> Nícolas
>>>
>>>> +		mtk_disp_pwm_update_bits(mdp, mdp->data->bls_debug,
>>>> +					 mdp->data->bls_debug_mask,
>>>> +					 mdp->data->bls_debug_mask);
>>>> +
>>>>    	rate = clk_get_rate(mdp->clk_main);
>>>>    	con0 = readl(mdp->base + mdp->data->con0);
>>>>    	con1 = readl(mdp->base + mdp->data->con1);
>>>> -- 
>>>> 2.39.0
>>>>
>>>>
>>
>>
diff mbox series

Patch

diff --git a/drivers/pwm/pwm-mtk-disp.c b/drivers/pwm/pwm-mtk-disp.c
index 82b430d881a2..fe9593f968ee 100644
--- a/drivers/pwm/pwm-mtk-disp.c
+++ b/drivers/pwm/pwm-mtk-disp.c
@@ -196,6 +196,16 @@  static int mtk_disp_pwm_get_state(struct pwm_chip *chip,
 		return err;
 	}
 
+	/*
+	 * Apply DISP_PWM_DEBUG settings to choose whether to enable or disable
+	 * registers double buffer and manual commit to working register before
+	 * performing any read/write operation
+	 */
+	if (mdp->data->bls_debug)
+		mtk_disp_pwm_update_bits(mdp, mdp->data->bls_debug,
+					 mdp->data->bls_debug_mask,
+					 mdp->data->bls_debug_mask);
+
 	rate = clk_get_rate(mdp->clk_main);
 	con0 = readl(mdp->base + mdp->data->con0);
 	con1 = readl(mdp->base + mdp->data->con1);