linux-kernel - Re: [PATCH v2] drm/bridge: Add debugfs print for bridge chains

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Date:   Tue, 25 Jul 2023 14:37:18 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc:     Andrzej Hajda <andrzej.hajda@...el.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Francesco Dolcini <francesco.dolcini@...adex.com>,
        Aradhya Bhatia <a-bhatia1@...com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm/bridge: Add debugfs print for bridge chains

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@...00/i2c@...00000/dsi@e:toshiba,tc358778
> 	bridge[2] type: 0, ops: 0x3, OF: /bus@...00/i2c@...10000/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@...asonboard.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@...sung.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

-- 
Regards,

Laurent Pinchart