Re: [PATCH] panel/simple-simple: Identify simple DPI panels using .compatible field

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hello Anusha,

thanks for quickly addressing this issue!

On Tue, 24 Jun 2025 15:13:05 -0500
Anusha Srivatsa <asrivats@redhat.com> wrote:

> Currently driver is checking for desc == &panel_dpi to do the DPI
> specific panel desc allocations. This looks hacky.
> 
> The panel allocation in panel_simple_probe() breaks due to not having
> the desc for DPI scenario. This patch does the following:
> 
> - Rename panel_dpi_probe() to panel_dpi_get_desc() and call it before
> panel allocation. panel_dpi_get_desc() returns a panel desc unlike
> panel_dpi_probe() which returned an int. This way driver has a known
> connector type while allocating the panel.
> - panel_dpi_get_desc() returns a panel desc
> - Add a simple helper is_panel_dpi() to identify a simple DPI panel from
> a simple panel based on .compatible field
> 
> Fixes: de04bb0089a9 ("drm/panel/panel-simple: Use the new allocation in place of devm_kzalloc()")

Please add a mention to the discussion:
Closes: https://lore.kernel.org/all/20250612081834.GA248237@francesco-nb/

and also:
Reported-by: Francesco Dolcini <francesco@dolcini.it>

> Suggested-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> Suggested-by: Maxime Ripard <mripard@kernel.org>
> Cc: Francesco Dolcini <francesco@dolcini.it>
> Cc: Luca Ceresoli <luca.ceresoli@bootlin.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Signed-off-by: Anusha Srivatsa <asrivats@redhat.com>
> ---
> Seeing the below trace due to the changes introduced by:
> Commit de04bb0089a9 ("drm/panel/panel-simple: Use the new allocation in place of devm_kzalloc()")
> 
> [   12.089274] ------------[ cut here ]------------
> [   12.089303] WARNING: CPU: 0 PID: 96 at drivers/gpu/drm/bridge/panel.c:377 devm_drm_of_get_bridge+0xac/0xb8
> [   12.130808] Modules linked in: v4l2_jpeg pwm_imx27(+) imx_vdoa gpu_sched panel_simple imx6_media(C) imx_media_common
> (C) videobuf2_dma_contig pwm_bl gpio_keys v4l2_mem2mem fuse ipv6 autofs4
> [   12.147774] CPU: 0 UID: 0 PID: 96 Comm: kworker/u8:3 Tainted: G         C          6.16.0-rc1+ #1 PREEMPT
> [   12.157446] Tainted: [C]=CRAP
> [   12.160418] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
> [   12.166953] Workqueue: events_unbound deferred_probe_work_func
> [   12.172805] Call trace:
> [   12.172815]  unwind_backtrace from show_stack+0x10/0x14
> [   12.180598]  show_stack from dump_stack_lvl+0x68/0x74
> [   12.185674]  dump_stack_lvl from __warn+0x7c/0xe0
> [   12.190407]  __warn from warn_slowpath_fmt+0x1b8/0x1c0
> [   12.195567]  warn_slowpath_fmt from devm_drm_of_get_bridge+0xac/0xb8
> [   12.201949]  devm_drm_of_get_bridge from imx_pd_probe+0x58/0x164
> [   12.207976]  imx_pd_probe from platform_probe+0x5c/0xb0
> [   12.213220]  platform_probe from really_probe+0xd0/0x3a4
> [   12.218551]  really_probe from __driver_probe_device+0x8c/0x1d4
> [   12.224486]  __driver_probe_device from driver_probe_device+0x30/0xc0
> [   12.230942]  driver_probe_device from __device_attach_driver+0x98/0x10c
> [   12.237572]  __device_attach_driver from bus_for_each_drv+0x90/0xe4
> [   12.243854]  bus_for_each_drv from __device_attach+0xa8/0x1c8
> [   12.249614]  __device_attach from bus_probe_device+0x88/0x8c
> [   12.255285]  bus_probe_device from deferred_probe_work_func+0x8c/0xcc
> [   12.261739]  deferred_probe_work_func from process_one_work+0x154/0x2dc
> [   12.268371]  process_one_work from worker_thread+0x250/0x3f0
> [   12.274043]  worker_thread from kthread+0x12c/0x24c
> [   12.278940]  kthread from ret_from_fork+0x14/0x28
> [   12.283660] Exception stack(0xd0be9fb0 to 0xd0be9ff8)
> [   12.288720] 9fa0:                                     00000000 00000000 00000000 00000000
> [   12.296906] 9fc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
> [   12.305089] 9fe0: 00000000 00000000 00000000 00000000 00000013 00000000
> [   12.312050] ---[ end trace 0000000000000000 ]---
> ---
>  drivers/gpu/drm/panel/panel-simple.c | 38 +++++++++++++++++++++---------------
>  1 file changed, 22 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index 0a3b26bb4d731c54614e24e38018c308acd5367a..2e6fd545100388a9d53183a5621e7b8fdb4148ae 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -26,6 +26,7 @@
>  #include <linux/i2c.h>
>  #include <linux/media-bus-format.h>
>  #include <linux/module.h>
> +#include <linux/of_device.h>
>  #include <linux/of_platform.h>
>  #include <linux/platform_device.h>
>  #include <linux/pm_runtime.h>
> @@ -432,8 +433,7 @@ static const struct drm_panel_funcs panel_simple_funcs = {
>  
>  static struct panel_desc panel_dpi;
>  
> -static int panel_dpi_probe(struct device *dev,
> -			   struct panel_simple *panel)
> +static struct panel_desc *panel_dpi_get_desc(struct device *dev)
>  {
>  	struct display_timing *timing;
>  	const struct device_node *np;
> @@ -445,17 +445,17 @@ static int panel_dpi_probe(struct device *dev,
>  	np = dev->of_node;
>  	desc = devm_kzalloc(dev, sizeof(*desc), GFP_KERNEL);
>  	if (!desc)
> -		return -ENOMEM;
> +		return NULL;
>  
>  	timing = devm_kzalloc(dev, sizeof(*timing), GFP_KERNEL);
>  	if (!timing)
> -		return -ENOMEM;
> +		return NULL;
>  
>  	ret = of_get_display_timing(np, "panel-timing", timing);
>  	if (ret < 0) {
>  		dev_err(dev, "%pOF: no panel-timing node found for \"panel-dpi\" binding\n",
>  			np);
> -		return ret;
> +		return NULL;
>  	}
>  
>  	desc->timings = timing;
> @@ -473,9 +473,7 @@ static int panel_dpi_probe(struct device *dev,
>  	/* We do not know the connector for the DT node, so guess it */
>  	desc->connector_type = DRM_MODE_CONNECTOR_DPI;
>  
> -	panel->desc = desc;
> -
> -	return 0;
> +	return desc;
>  }
>  
>  #define PANEL_SIMPLE_BOUNDS_CHECK(to_check, bounds, field) \
> @@ -570,6 +568,15 @@ static int panel_simple_override_nondefault_lvds_datamapping(struct device *dev,
>  	return 0;
>  }
>  
> +static bool is_panel_dpi(struct device *dev)
> +{
> +	const struct of_device_id *match;
> +
> +	match = of_match_device(dev->driver->of_match_table, dev);
> +
> +	return strcmp(match->compatible, "panel_dpi");

The compatible is panel-dpi, not panel_dpi.

> +}
> +
>  static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
>  {
>  	struct panel_simple *panel;
> @@ -579,6 +586,10 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
>  	u32 bus_flags;
>  	int err;
>  
> +	/* Is this simple panel a DPI panel */
> +	if (is_panel_dpi(dev))
> +		desc = panel_dpi_get_desc(dev);
> +
>  	panel = devm_drm_panel_alloc(dev, struct panel_simple, base,
>  				     &panel_simple_funcs, desc->connector_type);
>  	if (IS_ERR(panel))
> @@ -611,16 +622,11 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
>  			return -EPROBE_DEFER;
>  	}
>  
> -	if (desc == &panel_dpi) {
> -		/* Handle the generic panel-dpi binding */
> -		err = panel_dpi_probe(dev, panel);
> -		if (err)
> -			goto free_ddc;
> -		desc = panel->desc;
> -	} else {
> +	if (is_panel_dpi(dev))
> +		goto free_ddc;
> +	else
>  		if (!of_get_display_timing(dev->of_node, "panel-timing", &dt))
>  			panel_simple_parse_panel_timing_node(dev, panel, &dt);
> -	}

The 'else' becomes unnecessary after a 'if () goto xyz;'

With your changes you should now be able to remove both this entry in
platform_of_match:

	/* Must be the last entry */
	.compatible = "panel-dpi",
	.data = &panel_dpi,

and finally this line:

	static struct panel_desc panel_dpi;

Otherwise LGTM.

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com