linux-kernel - Re: [PATCH] drm/vkms: Fix bad matrix offset component multiplication

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Message-ID: <28d9f246-8952-4f03-94bc-60eadca78c2f@bootlin.com>
Date: Mon, 29 Dec 2025 10:33:53 +0100
From: Louis Chauvet <louis.chauvet@...tlin.com>
To: Ariel D'Alessandro <ariel.dalessandro@...labora.com>,
 Haneen Mohammed <hamohammed.sa@...il.com>, Simona Vetter <simona@...ll.ch>,
 Melissa Wen <melissa.srw@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 nfraprado@...labora.com, pekka.paalanen@...labora.com,
 daniels@...labora.com, kernel@...labora.com
Subject: Re: [PATCH] drm/vkms: Fix bad matrix offset component multiplication



On 12/23/25 18:26, Ariel D'Alessandro wrote:
> Pixels values are packed as 16-bit UNORM values, so the matrix offset
> components must be multiplied properly by the idempotent element -i.e.
> number 1 encoded as 16-bit UNORM-.
> 
> Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@...labora.com>

Hi,

Thanks for this patch!

How did you catch the issue? If you have an example of failing color 
conversion, can you add a test case in vkms_color_test?

Thanks,
Louis Chauvet

> ---
>   drivers/gpu/drm/vkms/vkms_composer.c | 13 ++++++++++---
>   1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index 3cf3f26e0d8ea..efdaf665435d9 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -16,6 +16,8 @@
>   #include "vkms_composer.h"
>   #include "vkms_luts.h"
>   
> +#define UNORM_16BIT_ONE			(1ULL << 16)
> +
>   static u16 pre_mul_blend_channel(u16 src, u16 dst, u16 alpha)
>   {
>   	u32 new_color;
> @@ -138,20 +140,25 @@ VISIBLE_IF_KUNIT void apply_3x4_matrix(struct pixel_argb_s32 *pixel,
>   	g = drm_int2fixp(pixel->g);
>   	b = drm_int2fixp(pixel->b);
>   
> +	/*
> +	 * Pixels values are packed as 16-bit UNORM values, so the matrix offset
> +	 * components must be multiplied properly by the idempotent element -i.e.
> +	 * number 1 encoded as 16-bit UNORM-.
> +	 */
>   	rf = drm_fixp_mul(drm_sm2fixp(matrix->matrix[0]), r) +
>   	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[1]), g) +
>   	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[2]), b) +
> -	     drm_sm2fixp(matrix->matrix[3]);
> +	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[3]), drm_int2fixp(UNORM_16BIT_ONE));
>   
>   	gf = drm_fixp_mul(drm_sm2fixp(matrix->matrix[4]), r) +
>   	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[5]), g) +
>   	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[6]), b) +
> -	     drm_sm2fixp(matrix->matrix[7]);
> +	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[7]), drm_int2fixp(UNORM_16BIT_ONE));
>   
>   	bf = drm_fixp_mul(drm_sm2fixp(matrix->matrix[8]), r) +
>   	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[9]), g) +
>   	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[10]), b) +
> -	     drm_sm2fixp(matrix->matrix[11]);
> +	     drm_fixp_mul(drm_sm2fixp(matrix->matrix[11]), drm_int2fixp(UNORM_16BIT_ONE));
>   
>   	pixel->r = drm_fixp2int_round(rf);
>   	pixel->g = drm_fixp2int_round(gf);
> 
> ---
> base-commit: b96bcfba104c65db41378a04f5ccac186f79578f
> change-id: 20251223-vkms-composer-fix-matrix-aa2c593f4515
> 
> Best regards,