Re: [PATCH v4 4/5] HACK: Always fallback

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] drm/xe/display: debug return false in xe_display_bo_fbdev_prefer_stolen()
--

commit 6c3a580cc3d1b156a2c5d921989c5c7a936adda3
Author: Maarten Lankhorst <dev@lankhorst.se>

HACK: Always fallback

This commit adds an unconditional return false to
xe_display_bo_fbdev_prefer_stolen() in the xe display driver.

> diff --git a/drivers/gpu/drm/xe/display/xe_display_bo.c b/drivers/gpu/drm/xe/display/xe_display_bo.c
> index 9385026a80b97..d78d893210c4f 100644
> --- a/drivers/gpu/drm/xe/display/xe_display_bo.c
> +++ b/drivers/gpu/drm/xe/display/xe_display_bo.c
> @@ -142,6 +142,8 @@ bool xe_display_bo_fbdev_prefer_stolen(struct xe_device *xe, unsigned int size)
>  	if (XE_DEVICE_WA(xe, 22019338487_display))
>  		return false;
>  
> +	return false;

[Severity: Medium]
As reported by Sashiko on v2 [1], is this temporary debugging commit
intended to be dropped before merging?

Adding an unconditional return false in xe_display_bo_fbdev_prefer_stolen()
in drivers/gpu/drm/xe/display/xe_display_bo.c makes the subsequent stolen
memory size check unreachable dead code and renders all earlier checks and
the size parameter unused:

drivers/gpu/drm/xe/display/xe_display_bo.c:xe_display_bo_fbdev_prefer_stolen() {
    ...
	if (XE_DEVICE_WA(xe, 22019338487_display))
		return false;

	return false;

	/*
	 * If the FB is too big, just don't use it since fbdev is not very
	 * important and we should probably use that space with FBC or other
	 * features.
	 */
	return stolen->size >= (size * 2) >> PAGE_SHIFT;
}

During display initialization on integrated platforms with
CONFIG_FRAMEBUFFER_CONSOLE and CONFIG_DRM_FBDEV_EMULATION enabled,
initial_plane_bo() in drivers/gpu/drm/xe/display/xe_initial_plane.c calls
xe_display_bo_fbdev_prefer_stolen():

drivers/gpu/drm/xe/display/xe_initial_plane.c:initial_plane_bo() {
    ...
		if (IS_ENABLED(CONFIG_FRAMEBUFFER_CONSOLE) &&
		    IS_ENABLED(CONFIG_DRM_FBDEV_EMULATION) &&
		    !xe_display_bo_fbdev_prefer_stolen(xe, plane_config->size)) {
			drm_info(&xe->drm, "Initial FB size exceeds half of stolen, discarding\n");
			return NULL;
		}
    ...
}

Because xe_display_bo_fbdev_prefer_stolen() now unconditionally returns
false, initial_plane_bo() always discards the initial BIOS framebuffer with
a misleading log message, and even with the subsequent takeover helper
xe_display_bo_fbdev_bios_fb_takeover() it will always fall back to
allocating a new system memory BO and blit-copying the BIOS framebuffer
instead of keeping it in stolen memory when eligible.

[1] https://lore.kernel.org/r/20260721142610.93804-11-dev@lankhorst.se

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261105161444.593259-7-dev@lankhorst.se?part=4