[v2] drm/bridge: Add debugfs print for bridge chains

  DRM bridges are not visible to the userspace and it may not be
immediately clear if the chain is somehow constructed incorrectly. I
have had two separate instances of a bridge driver failing to do a
drm_bridge_attach() call, resulting in the bridge connector not being
part of the chain. In some situations this doesn't seem to cause issues,
but it will if DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is used.

Add a debugfs file to print the bridge chains. For me, on this TI AM62
based platform, I get the following output:

encoder[39]
	bridge[0] type: 0, ops: 0x0
	bridge[1] type: 0, ops: 0x0, OF: /bus@f0000/i2c@20000000/dsi@e:toshiba,tc358778
	bridge[2] type: 0, ops: 0x3, OF: /bus@f0000/i2c@20010000/hdmi@48:lontium,lt8912b
	bridge[3] type: 11, ops: 0x7, OF: /hdmi-connector:hdmi-connector

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
Changes in v2:
- Fixed compilation issue when !CONFIG_OF
- Link to v1: https://lore.kernel.org/r/20230721-drm-bridge-chain-debugfs-v1-1-8614ff7e890d@ideasonboard.com
---
 drivers/gpu/drm/drm_bridge.c  | 50 +++++++++++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/drm_debugfs.c |  3 +++
 include/drm/drm_bridge.h      |  5 +++++
 3 files changed, 58 insertions(+)


---
base-commit: c7a472297169156252a50d76965eb36b081186e2
change-id: 20230721-drm-bridge-chain-debugfs-0bbc1522f57a

Best regards,
  

Hi Tomi,

Am Freitag, 21. Juli 2023, 17:01:39 CEST schrieb Tomi Valkeinen:
> DRM bridges are not visible to the userspace and it may not be
> immediately clear if the chain is somehow constructed incorrectly. I
> have had two separate instances of a bridge driver failing to do a
> drm_bridge_attach() call, resulting in the bridge connector not being
> part of the chain. In some situations this doesn't seem to cause issues,
> but it will if DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is used.
> 
> Add a debugfs file to print the bridge chains. For me, on this TI AM62
> based platform, I get the following output:
> 
> encoder[39]
> 	bridge[0] type: 0, ops: 0x0
> 	bridge[1] type: 0, ops: 0x0, OF:
> /bus@f0000/i2c@20000000/dsi@e:toshiba,tc358778 bridge[2] type: 0, ops: 0x3,
> OF: /bus@f0000/i2c@20010000/hdmi@48:lontium,lt8912b bridge[3] type: 11,
> ops: 0x7, OF: /hdmi-connector:hdmi-connector

I like the idea and it works on an imx8mp based board for the DRI display 
devices which have connectors attached:

$ cat /sys/kernel/debug/dri/1/bridge_chains 
encoder[36]
        bridge[0] type: 16, ops: 0x0, OF: /soc@0/bus@32c00000/
dsi@32e60000:fsl,imx8mp-mipi-dsim
        bridge[1] type: 10, ops: 0x3, OF: /soc@0/bus@30800000/i2c@30a30000/
bridge@f:toshiba,tc9595

$ cat /sys/kernel/debug/dri/2/bridge_chains 
encoder[36]
        bridge[0] type: 0, ops: 0x0, OF: /soc@0/bus@32c00000/display-
bridge@32fc4000:fsl,imx8mp-hdmi-pvi
        bridge[1] type: 0, ops: 0x7, OF: /soc@0/bus@32c00000/
hdmi@32fd8000:fsl,imx8mp-hdmi

Unfortunately this oopses on GPU device without connectors:

$ cat /sys/kernel/debug/dri/0/name 
etnaviv dev=etnaviv unique=etnaviv
$ cat /sys/kernel/debug/dri/0/bridge_chains
Unable to handle kernel NULL pointer dereference at virtual address 
0000000000000010
Mem abort info:
  ESR = 0x0000000096000004
  EC = 0x25: DABT (current EL), IL = 32 bits
  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
  FSC = 0x04: level 0 translation fault
Data abort info:
  ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
  CM = 0, WnR = 0, TnD = 0, TagAccess = 0
  GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
user pgtable: 4k pages, 48-bit VAs, pgdp=0000000045842000
[0000000000000010] pgd=0000000000000000, p4d=0000000000000000
Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
Modules linked in: dw_hdmi_cec mcp320x hantro_vpu snd_soc_fsl_asoc_card 
snd_soc_fsl_sai snd_soc_tlv320aic32x4_i2c snd_soc_imx_audmux snd_soc_fsl_utils 
snd_soc_tlv320aic32x4 snd_soc_simple_card_utils imx_pcm_dma snd_soc_core dw100 
v4l2_vp9 8021q snd_pcm_dmaengine v4l2_h264 snd_pcm videobuf2_dma_contig garp 
imx8mp_hdmi crct10dif_ce v4l2_mem2mem mrp dw_hdmi tc358767 videobuf2_memops 
stp bluetooth llc synopsys_edac videobuf2_v4l2 cec videobuf2_common 
phy_fsl_samsung_hdmi imx8mp_hdmi_pvi samsung_dsim snd_timer cfg80211 snd 
flexcan drm_display_helper imx_sdma soundcore virt_dma can_dev 
clk_renesas_pcie coresight_etm4x coresight_tmc coresight_funnel pwm_imx27 
coresight imx8mm_thermal iio_hwmon fuse ipv6
CPU: 3 PID: 485 Comm: cat Not tainted 6.5.0-rc3-next-20230724+ #1822 
41dd31be02d36f174370d905469174492535c29d
Hardware name: TQ-Systems i.MX8MPlus TQMa8MPxL on MBa8MPxL (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : drm_bridge_chains_info+0xfc/0x1d0
lr : drm_bridge_chains_info+0xf8/0x1d0
sp : ffff8000831c39e0
x29: ffff8000831c39e0 x28: ffff8000811759f0 x27: ffff800081175a08
x26: ffff800081175000 x25: ffff000002d592b0 x24: ffff000002d59000
x23: ffff800081175a70 x22: ffff800081175a50 x21: fffffffffffffff8
x20: 0000000000000000 x19: 0000000000000000 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
x14: 0000000000000000 x13: 3836613136616661 x12: 3030303030303030
x11: 203a7473696c5f72 x10: 65646f636e65205d x9 : 6c5f7265646f636e
x8 : 65205d6d72645b20 x7 : 0000000000000003 x6 : ffff8000818627a0
x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 0000000000000000 x0 : ffff8000831c3a68
Call trace:
 drm_bridge_chains_info+0xfc/0x1d0
 seq_read_iter+0x1a8/0x484
 seq_read+0x88/0xbc
 full_proxy_read+0x5c/0xa8
 vfs_read+0xac/0x1e0
 ksys_read+0x68/0xf4
 __arm64_sys_read+0x18/0x20
 invoke_syscall+0x6c/0xec
 el0_svc_common.constprop.0+0xb8/0xd8
 do_el0_svc+0x28/0x34
 el0_svc+0x1c/0x50
 el0t_64_sync_handler+0xb8/0xbc
 el0t_64_sync+0x14c/0x150
Code: aa1903e2 f94037e1 9401d66f 910223e0 (b9401aa2) 
---[ end trace 0000000000000000 ]---

This boils down to the connector_list setup incorrectly? Here is some debug 
output from drm_bridge_chains_info:
> etnaviv etnaviv: [drm] encoder_list: 00000000afa61a68
> etnaviv etnaviv: [drm] num_encoder: 0
> etnaviv etnaviv: [drm] list_empty: 0
> etnaviv etnaviv: [drm] encoder: fffffffffffffff8
> etnaviv etnaviv: [drm] encoder->head: 0000000000000000

list_empty(&config->encoder_list)) not returning true and num_encoder being 0 
seems off to me.

Best regards,
Alexander

> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> ---
> Changes in v2:
> - Fixed compilation issue when !CONFIG_OF
> - Link to v1:
> https://lore.kernel.org/r/20230721-drm-bridge-chain-debugfs-v1-1-8614ff7e89
> 0d@ideasonboard.com ---
>  drivers/gpu/drm/drm_bridge.c  | 50
> +++++++++++++++++++++++++++++++++++++++++++ drivers/gpu/drm/drm_debugfs.c |
>  3 +++
>  include/drm/drm_bridge.h      |  5 +++++
>  3 files changed, 58 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index c3d69af02e79..d3eb62d5ef3b 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -27,8 +27,10 @@
>  #include <linux/mutex.h>
> 
>  #include <drm/drm_atomic_state_helper.h>
> +#include <drm/drm_debugfs.h>
>  #include <drm/drm_bridge.h>
>  #include <drm/drm_encoder.h>
> +#include <drm/drm_file.h>
>  #include <drm/drm_of.h>
>  #include <drm/drm_print.h>
> 
> @@ -1345,6 +1347,54 @@ struct drm_bridge *of_drm_find_bridge(struct
> device_node *np) EXPORT_SYMBOL(of_drm_find_bridge);
>  #endif
> 
> +#ifdef CONFIG_DEBUG_FS
> +static int drm_bridge_chains_info(struct seq_file *m, void *data)
> +{
> +	struct drm_debugfs_entry *entry = m->private;
> +	struct drm_device *dev = entry->dev;
> +	struct drm_printer p = drm_seq_file_printer(m);
> +	struct drm_mode_config *config = &dev->mode_config;
> +	struct drm_encoder *encoder;
> +	unsigned int bridge_idx = 0;
> +
> +	list_for_each_entry(encoder, &config->encoder_list, head) {
> +		struct drm_bridge *bridge;
> +
> +		drm_printf(&p, "encoder[%u]\n", encoder->base.id);
> +
> +		bridge = drm_bridge_chain_get_first_bridge(encoder);
> +
> +		while (bridge) {
> +			drm_printf(&p, "\tbridge[%u] type: %u, ops: 
%#x",
> +				   bridge_idx, bridge->type, bridge-
>ops);
> +
> +#ifdef CONFIG_OF
> +			if (bridge->of_node)
> +				drm_printf(&p, ", OF: %pOFfc", bridge-
>of_node);
> +#endif
> +
> +			drm_printf(&p, "\n");
> +
> +			bridge_idx++;
> +			bridge = drm_bridge_get_next_bridge(bridge);
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +/* any use in debugfs files to dump individual planes/crtc/etc? */
> +static const struct drm_debugfs_info drm_bridge_debugfs_list[] = {
> +	{"bridge_chains", drm_bridge_chains_info, 0},
> +};
> +
> +void drm_bridge_debugfs_init(struct drm_minor *minor)
> +{
> +	drm_debugfs_add_files(minor->dev, drm_bridge_debugfs_list,
> +			      ARRAY_SIZE(drm_bridge_debugfs_list));
> +}
> +#endif
> +
>  MODULE_AUTHOR("Ajay Kumar <ajaykumar.rs@samsung.com>");
>  MODULE_DESCRIPTION("DRM bridge infrastructure");
>  MODULE_LICENSE("GPL and additional rights");
> diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
> index c90dbcffa0dc..3e89559d68cd 100644
> --- a/drivers/gpu/drm/drm_debugfs.c
> +++ b/drivers/gpu/drm/drm_debugfs.c
> @@ -31,6 +31,7 @@
> 
>  #include <drm/drm_atomic.h>
>  #include <drm/drm_auth.h>
> +#include <drm/drm_bridge.h>
>  #include <drm/drm_client.h>
>  #include <drm/drm_debugfs.h>
>  #include <drm/drm_device.h>
> @@ -272,6 +273,8 @@ int drm_debugfs_init(struct drm_minor *minor, int
> minor_id,
> 
>  	drm_debugfs_add_files(minor->dev, drm_debugfs_list, 
DRM_DEBUGFS_ENTRIES);
> 
> +	drm_bridge_debugfs_init(minor);
> +
>  	if (drm_drv_uses_atomic_modeset(dev)) {
>  		drm_atomic_debugfs_init(minor);
>  	}
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index bf964cdfb330..60dbee6bd1e6 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -949,4 +949,9 @@ static inline struct drm_bridge
> *drmm_of_get_bridge(struct drm_device *drm, }
>  #endif
> 
> +#ifdef CONFIG_DEBUG_FS
> +struct drm_minor;
> +void drm_bridge_debugfs_init(struct drm_minor *minor);
> +#endif
> +
>  #endif
> 
> ---
> base-commit: c7a472297169156252a50d76965eb36b081186e2
> change-id: 20230721-drm-bridge-chain-debugfs-0bbc1522f57a
> 
> Best regards,
  

Hi Tomi,

Thank you for the patch.

On Fri, Jul 21, 2023 at 06:01:39PM +0300, Tomi Valkeinen wrote:
> DRM bridges are not visible to the userspace and it may not be
> immediately clear if the chain is somehow constructed incorrectly. I
> have had two separate instances of a bridge driver failing to do a
> drm_bridge_attach() call, resulting in the bridge connector not being
> part of the chain. In some situations this doesn't seem to cause issues,
> but it will if DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is used.
> 
> Add a debugfs file to print the bridge chains. For me, on this TI AM62
> based platform, I get the following output:
> 
> encoder[39]
> 	bridge[0] type: 0, ops: 0x0
> 	bridge[1] type: 0, ops: 0x0, OF: /bus@f0000/i2c@20000000/dsi@e:toshiba,tc358778
> 	bridge[2] type: 0, ops: 0x3, OF: /bus@f0000/i2c@20010000/hdmi@48:lontium,lt8912b
> 	bridge[3] type: 11, ops: 0x7, OF: /hdmi-connector:hdmi-connector

Names would be more readable than numbers, but I'm not sure that's
really worth it. It can always be improved on top if desired.

> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> ---
> Changes in v2:
> - Fixed compilation issue when !CONFIG_OF
> - Link to v1: https://lore.kernel.org/r/20230721-drm-bridge-chain-debugfs-v1-1-8614ff7e890d@ideasonboard.com
> ---
>  drivers/gpu/drm/drm_bridge.c  | 50 +++++++++++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/drm_debugfs.c |  3 +++
>  include/drm/drm_bridge.h      |  5 +++++
>  3 files changed, 58 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> index c3d69af02e79..d3eb62d5ef3b 100644
> --- a/drivers/gpu/drm/drm_bridge.c
> +++ b/drivers/gpu/drm/drm_bridge.c
> @@ -27,8 +27,10 @@
>  #include <linux/mutex.h>
>  
>  #include <drm/drm_atomic_state_helper.h>
> +#include <drm/drm_debugfs.h>
>  #include <drm/drm_bridge.h>
>  #include <drm/drm_encoder.h>
> +#include <drm/drm_file.h>
>  #include <drm/drm_of.h>
>  #include <drm/drm_print.h>
>  
> @@ -1345,6 +1347,54 @@ struct drm_bridge *of_drm_find_bridge(struct device_node *np)
>  EXPORT_SYMBOL(of_drm_find_bridge);
>  #endif
>  
> +#ifdef CONFIG_DEBUG_FS
> +static int drm_bridge_chains_info(struct seq_file *m, void *data)
> +{
> +	struct drm_debugfs_entry *entry = m->private;
> +	struct drm_device *dev = entry->dev;
> +	struct drm_printer p = drm_seq_file_printer(m);
> +	struct drm_mode_config *config = &dev->mode_config;

As Alexander reported, there's a crash for GPU drivers, as mode_config
isn't initialized in that case. I would skip creating the debugfs entry
if DRIVER_MODESET isn't set.

> +	struct drm_encoder *encoder;
> +	unsigned int bridge_idx = 0;
> +
> +	list_for_each_entry(encoder, &config->encoder_list, head) {
> +		struct drm_bridge *bridge;
> +
> +		drm_printf(&p, "encoder[%u]\n", encoder->base.id);
> +
> +		bridge = drm_bridge_chain_get_first_bridge(encoder);
> +
> +		while (bridge) {

Would drm_for_each_bridge_in_chain() help ?

> +			drm_printf(&p, "\tbridge[%u] type: %u, ops: %#x",
> +				   bridge_idx, bridge->type, bridge->ops);
> +
> +#ifdef CONFIG_OF
> +			if (bridge->of_node)
> +				drm_printf(&p, ", OF: %pOFfc", bridge->of_node);
> +#endif
> +
> +			drm_printf(&p, "\n");
> +
> +			bridge_idx++;
> +			bridge = drm_bridge_get_next_bridge(bridge);
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +/* any use in debugfs files to dump individual planes/crtc/etc? */

Those can easily be listed from userspace, so I don't think that's
needed.

> +static const struct drm_debugfs_info drm_bridge_debugfs_list[] = {
> +	{"bridge_chains", drm_bridge_chains_info, 0},

Missing spaces after '{' and before '}'.

> +};
> +
> +void drm_bridge_debugfs_init(struct drm_minor *minor)
> +{
> +	drm_debugfs_add_files(minor->dev, drm_bridge_debugfs_list,
> +			      ARRAY_SIZE(drm_bridge_debugfs_list));
> +}
> +#endif
> +
>  MODULE_AUTHOR("Ajay Kumar <ajaykumar.rs@samsung.com>");
>  MODULE_DESCRIPTION("DRM bridge infrastructure");
>  MODULE_LICENSE("GPL and additional rights");
> diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
> index c90dbcffa0dc..3e89559d68cd 100644
> --- a/drivers/gpu/drm/drm_debugfs.c
> +++ b/drivers/gpu/drm/drm_debugfs.c
> @@ -31,6 +31,7 @@
>  
>  #include <drm/drm_atomic.h>
>  #include <drm/drm_auth.h>
> +#include <drm/drm_bridge.h>
>  #include <drm/drm_client.h>
>  #include <drm/drm_debugfs.h>
>  #include <drm/drm_device.h>
> @@ -272,6 +273,8 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>  
>  	drm_debugfs_add_files(minor->dev, drm_debugfs_list, DRM_DEBUGFS_ENTRIES);
>  
> +	drm_bridge_debugfs_init(minor);
> +
>  	if (drm_drv_uses_atomic_modeset(dev)) {
>  		drm_atomic_debugfs_init(minor);
>  	}
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index bf964cdfb330..60dbee6bd1e6 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -949,4 +949,9 @@ static inline struct drm_bridge *drmm_of_get_bridge(struct drm_device *drm,
>  }
>  #endif
>  
> +#ifdef CONFIG_DEBUG_FS

You could drop the conditional compilation, it wouldn't hurt.

> +struct drm_minor;
> +void drm_bridge_debugfs_init(struct drm_minor *minor);
> +#endif
> +
>  #endif
> 
> ---
> base-commit: c7a472297169156252a50d76965eb36b081186e2
> change-id: 20230721-drm-bridge-chain-debugfs-0bbc1522f57a
  

On 25/07/2023 14:37, Laurent Pinchart wrote:
> Hi Tomi,
> 
> Thank you for the patch.
> 
> On Fri, Jul 21, 2023 at 06:01:39PM +0300, Tomi Valkeinen wrote:
>> DRM bridges are not visible to the userspace and it may not be
>> immediately clear if the chain is somehow constructed incorrectly. I
>> have had two separate instances of a bridge driver failing to do a
>> drm_bridge_attach() call, resulting in the bridge connector not being
>> part of the chain. In some situations this doesn't seem to cause issues,
>> but it will if DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is used.
>>
>> Add a debugfs file to print the bridge chains. For me, on this TI AM62
>> based platform, I get the following output:
>>
>> encoder[39]
>> 	bridge[0] type: 0, ops: 0x0
>> 	bridge[1] type: 0, ops: 0x0, OF: /bus@f0000/i2c@20000000/dsi@e:toshiba,tc358778
>> 	bridge[2] type: 0, ops: 0x3, OF: /bus@f0000/i2c@20010000/hdmi@48:lontium,lt8912b
>> 	bridge[3] type: 11, ops: 0x7, OF: /hdmi-connector:hdmi-connector
> 
> Names would be more readable than numbers, but I'm not sure that's
> really worth it. It can always be improved on top if desired.

For type and ops? I agree, but it might also make the output more 
cluttered. To be honest, I'm not sure if type and ops are useful here, I 
just felt that I should print something else than just the OF node =).

>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> ---
>> Changes in v2:
>> - Fixed compilation issue when !CONFIG_OF
>> - Link to v1: https://lore.kernel.org/r/20230721-drm-bridge-chain-debugfs-v1-1-8614ff7e890d@ideasonboard.com
>> ---
>>   drivers/gpu/drm/drm_bridge.c  | 50 +++++++++++++++++++++++++++++++++++++++++++
>>   drivers/gpu/drm/drm_debugfs.c |  3 +++
>>   include/drm/drm_bridge.h      |  5 +++++
>>   3 files changed, 58 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
>> index c3d69af02e79..d3eb62d5ef3b 100644
>> --- a/drivers/gpu/drm/drm_bridge.c
>> +++ b/drivers/gpu/drm/drm_bridge.c
>> @@ -27,8 +27,10 @@
>>   #include <linux/mutex.h>
>>   
>>   #include <drm/drm_atomic_state_helper.h>
>> +#include <drm/drm_debugfs.h>
>>   #include <drm/drm_bridge.h>
>>   #include <drm/drm_encoder.h>
>> +#include <drm/drm_file.h>
>>   #include <drm/drm_of.h>
>>   #include <drm/drm_print.h>
>>   
>> @@ -1345,6 +1347,54 @@ struct drm_bridge *of_drm_find_bridge(struct device_node *np)
>>   EXPORT_SYMBOL(of_drm_find_bridge);
>>   #endif
>>   
>> +#ifdef CONFIG_DEBUG_FS
>> +static int drm_bridge_chains_info(struct seq_file *m, void *data)
>> +{
>> +	struct drm_debugfs_entry *entry = m->private;
>> +	struct drm_device *dev = entry->dev;
>> +	struct drm_printer p = drm_seq_file_printer(m);
>> +	struct drm_mode_config *config = &dev->mode_config;
> 
> As Alexander reported, there's a crash for GPU drivers, as mode_config
> isn't initialized in that case. I would skip creating the debugfs entry
> if DRIVER_MODESET isn't set.

Yes, makes sense.

>> +	struct drm_encoder *encoder;
>> +	unsigned int bridge_idx = 0;
>> +
>> +	list_for_each_entry(encoder, &config->encoder_list, head) {
>> +		struct drm_bridge *bridge;
>> +
>> +		drm_printf(&p, "encoder[%u]\n", encoder->base.id);
>> +
>> +		bridge = drm_bridge_chain_get_first_bridge(encoder);
>> +
>> +		while (bridge) {
> 
> Would drm_for_each_bridge_in_chain() help ?

Yes.

>> +			drm_printf(&p, "\tbridge[%u] type: %u, ops: %#x",
>> +				   bridge_idx, bridge->type, bridge->ops);
>> +
>> +#ifdef CONFIG_OF
>> +			if (bridge->of_node)
>> +				drm_printf(&p, ", OF: %pOFfc", bridge->of_node);
>> +#endif
>> +
>> +			drm_printf(&p, "\n");
>> +
>> +			bridge_idx++;
>> +			bridge = drm_bridge_get_next_bridge(bridge);
>> +		}
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +/* any use in debugfs files to dump individual planes/crtc/etc? */
> 
> Those can easily be listed from userspace, so I don't think that's
> needed.

Oops. That comment is not supposed to be there. It was a copy-paste error.

>> +static const struct drm_debugfs_info drm_bridge_debugfs_list[] = {
>> +	{"bridge_chains", drm_bridge_chains_info, 0},
> 
> Missing spaces after '{' and before '}'.

Yep.

>> +};
>> +
>> +void drm_bridge_debugfs_init(struct drm_minor *minor)
>> +{
>> +	drm_debugfs_add_files(minor->dev, drm_bridge_debugfs_list,
>> +			      ARRAY_SIZE(drm_bridge_debugfs_list));
>> +}
>> +#endif
>> +
>>   MODULE_AUTHOR("Ajay Kumar <ajaykumar.rs@samsung.com>");
>>   MODULE_DESCRIPTION("DRM bridge infrastructure");
>>   MODULE_LICENSE("GPL and additional rights");
>> diff --git a/drivers/gpu/drm/drm_debugfs.c b/drivers/gpu/drm/drm_debugfs.c
>> index c90dbcffa0dc..3e89559d68cd 100644
>> --- a/drivers/gpu/drm/drm_debugfs.c
>> +++ b/drivers/gpu/drm/drm_debugfs.c
>> @@ -31,6 +31,7 @@
>>   
>>   #include <drm/drm_atomic.h>
>>   #include <drm/drm_auth.h>
>> +#include <drm/drm_bridge.h>
>>   #include <drm/drm_client.h>
>>   #include <drm/drm_debugfs.h>
>>   #include <drm/drm_device.h>
>> @@ -272,6 +273,8 @@ int drm_debugfs_init(struct drm_minor *minor, int minor_id,
>>   
>>   	drm_debugfs_add_files(minor->dev, drm_debugfs_list, DRM_DEBUGFS_ENTRIES);
>>   
>> +	drm_bridge_debugfs_init(minor);
>> +
>>   	if (drm_drv_uses_atomic_modeset(dev)) {
>>   		drm_atomic_debugfs_init(minor);
>>   	}
>> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
>> index bf964cdfb330..60dbee6bd1e6 100644
>> --- a/include/drm/drm_bridge.h
>> +++ b/include/drm/drm_bridge.h
>> @@ -949,4 +949,9 @@ static inline struct drm_bridge *drmm_of_get_bridge(struct drm_device *drm,
>>   }
>>   #endif
>>   
>> +#ifdef CONFIG_DEBUG_FS
> 
> You could drop the conditional compilation, it wouldn't hurt.

I used the same style as in drm_crtc_internal.h. But you're right, the 
ifdef doesn't really do much here.

>> +struct drm_minor;
>> +void drm_bridge_debugfs_init(struct drm_minor *minor);
>> +#endif
>> +
>>   #endif
>>
>> ---
>> base-commit: c7a472297169156252a50d76965eb36b081186e2
>> change-id: 20230721-drm-bridge-chain-debugfs-0bbc1522f57a
> 

  Tomi