linux-kernel - Re: [PATCH V3 4/5] drm/vkms: Compute CRC without change input data

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Message-ID: <20190711082105.GI15868@phenom.ffwll.local>
Date:   Thu, 11 Jul 2019 10:21:05 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
Cc:     Brian Starkey <brian.starkey@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        Simon Ser <contact@...rsion.fr>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 4/5] drm/vkms: Compute CRC without change input data

On Tue, Jun 25, 2019 at 10:38:31PM -0300, Rodrigo Siqueira wrote:
> The compute_crc() function is responsible for calculating the
> framebuffer CRC value; due to the XRGB format, this function has to
> ignore the alpha channel during the CRC computation. Therefore,
> compute_crc() set zero to the alpha channel directly in the input
> framebuffer, which is not a problem since this function receives a copy
> of the original buffer. However, if we want to use this function in a
> context without a buffer copy, it will change the initial value. This
> patch makes compute_crc() calculate the CRC value without modifying the
> input framebuffer.

Uh why? For writeback we're writing the output too, so we can write
whatever we want to into the alpha channel. For writeback we should never
accept a pixel format where alpha actually matters, that doesn't make
sense. You can't see through a real screen either, they are all opaque :-)
-Daniel

> 
> Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
> ---
>  drivers/gpu/drm/vkms/vkms_composer.c | 31 +++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
> index 51a270514219..8126aa0f968f 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -6,33 +6,40 @@
>  #include <drm/drm_atomic_helper.h>
>  #include <drm/drm_gem_framebuffer_helper.h>
>  
> +static u32 get_pixel_from_buffer(int x, int y, const u8 *buffer,
> +				 const struct vkms_composer *composer)
> +{
> +	int src_offset = composer->offset + (y * composer->pitch)
> +					  + (x * composer->cpp);
> +
> +	return *(u32 *)&buffer[src_offset];
> +}
> +
>  /**
>   * compute_crc - Compute CRC value on output frame
>   *
> - * @vaddr_out: address to final framebuffer
> + * @vaddr: address to final framebuffer
>   * @composer: framebuffer's metadata
>   *
>   * returns CRC value computed using crc32 on the visible portion of
>   * the final framebuffer at vaddr_out
>   */
> -static uint32_t compute_crc(void *vaddr_out, struct vkms_composer *composer)
> +static uint32_t compute_crc(const u8 *vaddr,
> +			    const struct vkms_composer *composer)
>  {
> -	int i, j, src_offset;
> +	int x, y;
>  	int x_src = composer->src.x1 >> 16;
>  	int y_src = composer->src.y1 >> 16;
>  	int h_src = drm_rect_height(&composer->src) >> 16;
>  	int w_src = drm_rect_width(&composer->src) >> 16;
> -	u32 crc = 0;
> +	u32 crc = 0, pixel = 0;
>  
> -	for (i = y_src; i < y_src + h_src; ++i) {
> -		for (j = x_src; j < x_src + w_src; ++j) {
> -			src_offset = composer->offset
> -				     + (i * composer->pitch)
> -				     + (j * composer->cpp);
> +	for (y = y_src; y < y_src + h_src; ++y) {
> +		for (x = x_src; x < x_src + w_src; ++x) {
>  			/* XRGB format ignores Alpha channel */
> -			memset(vaddr_out + src_offset + 24, 0,  8);
> -			crc = crc32_le(crc, vaddr_out + src_offset,
> -				       sizeof(u32));
> +			pixel = get_pixel_from_buffer(x, y, vaddr, composer);
> +			bitmap_clear((void *)&pixel, 0, 8);
> +			crc = crc32_le(crc, (void *)&pixel, sizeof(u32));
>  		}
>  	}
>  
> -- 
> 2.21.0

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch