Re: [PATCH v2 3/5] drm/rcar-du: dsc: Add rudimentary Renesas R-Car V4H DSC driver

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:
- [High] Sleeping functions called from atomic bridge callbacks (Subsystem Guide Violation).
- [Medium] Resource leak of the remote bridge module reference.
- [Low] Kconfig logic error defeats COMPILE_TEST support.
- [Low] Unbalanced clock management due to ignored initialization errors.
--

commit bddd7270b2102e27ba00c2ae1f77dd8387606431
Author: Marek Vasut <marek.vasut+renesas@mailbox.org>

drm/rcar-du: dsc: Add rudimentary Renesas R-Car V4H DSC driver

The Renesas DSC Display Stream Compression is a bridge embedded in the
Renesas R-Car V4H SoC. The bridge performs VESA DSC encoding of up to
8k or 400 Mpixel/s. Add rudimentary driver to act as a pass-through
bridge for DSI1.

> diff --git a/drivers/gpu/drm/renesas/rcar-du/Kconfig b/drivers/gpu/drm/renesas/rcar-du/Kconfig
> index 840305fdeb49a..a51f996d35375 100644
> --- a/drivers/gpu/drm/renesas/rcar-du/Kconfig
> +++ b/drivers/gpu/drm/renesas/rcar-du/Kconfig
> @@ -33,6 +33,19 @@ config DRM_RCAR_DW_HDMI
>  	help
>  	  Enable support for R-Car Gen3 or RZ/G2 internal HDMI encoder.
>  
> +config DRM_RCAR_USE_DSC
> +	bool "R-Car DU DSC Encoder Support"
> +	depends on DRM_BRIDGE && OF
> +	depends on DRM_RCAR_DU || COMPILE_TEST
> +	default DRM_RCAR_DU
> +	help
> +	  Enable support for the R-Car Display Unit embedded DSC encoder.
> +
> +config DRM_RCAR_DSC
> +	def_tristate DRM_RCAR_DU
> +	depends on DRM_RCAR_USE_DSC
> +	select RESET_CONTROLLER

Since DRM_RCAR_DSC lacks a user prompt and uses def_tristate DRM_RCAR_DU,
doesn't it silently evaluate to 'n' during cross-architecture compile tests
where DRM_RCAR_DU=n?

This seems to prevent rcar_dsc.c from being compiled even when COMPILE_TEST
is enabled in DRM_RCAR_USE_DSC.

> diff --git a/drivers/gpu/drm/renesas/rcar-du/rcar_dsc.c b/drivers/gpu/drm/renesas/rcar-du/rcar_dsc.c
> new file mode 100644
> index 0000000000000..2ab5a4bb099c3
> --- /dev/null
> +++ b/drivers/gpu/drm/renesas/rcar-du/rcar_dsc.c
[ ... ]
> +static void rcar_dsc_atomic_enable(struct drm_bridge *bridge,
> +				   struct drm_atomic_state *state)
> +{
> +	struct rcar_dsc *dsc = bridge_to_rcar_dsc(bridge);
> +
> +	WARN_ON(clk_prepare_enable(dsc->clk));

Does this sequence execute in atomic context?

Looking at the DRM subsystem guidelines, bridge atomic enable/disable
callbacks run in atomic context where sleeping is forbidden. Since
clk_prepare_enable() acquires a mutex internally, can this trigger a
scheduling while atomic warning?

Also, if clk_prepare_enable() fails, the failure is recorded by WARN_ON()
but execution continues. Later, the pipeline teardown will unconditionally
call clk_disable_unprepare().

Can this lead to unbalanced clock disable warnings from the Common Clock
Framework since the clock was never actually enabled?

> +	WARN_ON(reset_control_deassert(dsc->rst));
> +}
> +
> +static void rcar_dsc_atomic_disable(struct drm_bridge *bridge,
> +				    struct drm_atomic_state *state)
> +{
> +	struct rcar_dsc *dsc = bridge_to_rcar_dsc(bridge);
> +
> +	reset_control_assert(dsc->rst);
> +	clk_disable_unprepare(dsc->clk);
> +}
[ ... ]
> +static int rcar_dsc_probe(struct platform_device *pdev)
> +{
[ ... ]
> +	dsc->bridge.next_bridge = of_drm_find_and_get_bridge(remote);
> +	of_node_put(remote);
> +	if (!dsc->bridge.next_bridge)
> +		return -EPROBE_DEFER;
> +
> +	dsc->bridge.of_node = dev->of_node;
> +
> +	return devm_drm_bridge_add(dev, &dsc->bridge);
> +}

Does this code leak the remote bridge module reference?

The call to of_drm_find_and_get_bridge() acquires a reference to the
bridge that requires a matching drm_bridge_put(). If devm_drm_bridge_add()
fails, the reference doesn't appear to be released.

Additionally, there doesn't seem to be a .remove callback or a registered
devm action to release the reference when the driver unbinds. Would using
devm_drm_of_get_bridge() resolve this?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260515-rcar-du-dsc-v2-0-f6b9240a1240@ideasonboard.com?part=3