linux-kernel - Re: [PATCH V4] drm/ast: Swap framebuffer writes on big-endian machines

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Message-ID: <007be221-b28d-49c0-a7c7-0a4ea74d5d75@suse.de>
Date: Mon, 15 Dec 2025 08:30:06 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: René Rebe <rene@...ctco.de>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc: Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH V4] drm/ast: Swap framebuffer writes on big-endian
 machines



Am 12.12.25 um 21:05 schrieb René Rebe:
> Swap the pixel data when writing to framebuffer memory on big-endian
> machines. Fixes incorrect output. Aspeed graphics does not appear to
> support big-endian framebuffers after AST2400, although the feature
> has been documented.
>
> There's a lengthy discussion at [1].
>
> Signed-off-by: René Rebe <rene@...ctco.de>
> Link: https://lore.kernel.org/dri-devel/20251202.170626.2134482663677806825.rene@exactco.de/ # [1]

Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>

Thanks for the updated patch.

> ---
> The ARGB4444 cursor not yet 100% correct, but that might be another layer/helper bug.
> Tested on sparc64 T4-1 running T2/Linux.
> ---
>   drivers/gpu/drm/ast/ast_cursor.c | 11 ++++++++---
>   drivers/gpu/drm/ast/ast_mode.c   | 11 +++++++++--
>   2 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/ast/ast_cursor.c b/drivers/gpu/drm/ast/ast_cursor.c
> index 2d3ad7610c2e..d4620171d845 100644
> --- a/drivers/gpu/drm/ast/ast_cursor.c
> +++ b/drivers/gpu/drm/ast/ast_cursor.c
> @@ -92,12 +92,17 @@ static void ast_set_cursor_image(struct ast_device *ast, const u8 *src,
>   				 unsigned int width, unsigned int height)
>   {
>   	u8 __iomem *dst = ast_plane_vaddr(&ast->cursor_plane.base);
> -	u32 csum;
> -
> -	csum = ast_cursor_calculate_checksum(src, width, height);
> +	u32 csum = ast_cursor_calculate_checksum(src, width, height);
>   
>   	/* write pixel data */
> +#if defined(__BIG_ENDIAN)
> +	unsigned int i;
> +
> +	for (i = 0; i < AST_HWC_SIZE; i += 2)
> +		writew(swab16(*(const __be16 *)&src[i]), &dst[i]);
> +#else
>   	memcpy_toio(dst, src, AST_HWC_SIZE);
> +#endif
>   
>   	/* write checksum + signature */
>   	dst += AST_HWC_SIZE;
> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
> index cd08990a10f9..57c6fbc3232b 100644
> --- a/drivers/gpu/drm/ast/ast_mode.c
> +++ b/drivers/gpu/drm/ast/ast_mode.c
> @@ -526,12 +526,18 @@ static int ast_primary_plane_helper_atomic_check(struct drm_plane *plane,
>   
>   static void ast_handle_damage(struct ast_plane *ast_plane, struct iosys_map *src,
>   			      struct drm_framebuffer *fb,
> -			      const struct drm_rect *clip)
> +			      const struct drm_rect *clip,
> +			      struct drm_format_conv_state *fmtcnv_state)
>   {
>   	struct iosys_map dst = IOSYS_MAP_INIT_VADDR_IOMEM(ast_plane_vaddr(ast_plane));
>   
>   	iosys_map_incr(&dst, drm_fb_clip_offset(fb->pitches[0], fb->format, clip));
> +
> +#if defined(__BIG_ENDIAN)
> +	drm_fb_swab(&dst, fb->pitches, src, fb, clip, !src[0].is_iomem, fmtcnv_state);
> +#else
>   	drm_fb_memcpy(&dst, fb->pitches, src, fb, clip);
> +#endif
>   }
>   
>   static void ast_primary_plane_helper_atomic_update(struct drm_plane *plane,
> @@ -561,7 +567,8 @@ static void ast_primary_plane_helper_atomic_update(struct drm_plane *plane,
>   	if (drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE) == 0) {
>   		drm_atomic_helper_damage_iter_init(&iter, old_plane_state, plane_state);
>   		drm_atomic_for_each_plane_damage(&iter, &damage) {
> -			ast_handle_damage(ast_plane, shadow_plane_state->data, fb, &damage);
> +			ast_handle_damage(ast_plane, shadow_plane_state->data, fb, &damage,
> +					  &shadow_plane_state->fmtcnv_state);
>   		}
>   
>   		drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)