linux-kernel - Re: [PATCH V4 2/3] drm/vkms: Compute CRC without change input data

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Date:   Tue, 12 May 2020 12:34:47 +0100
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Rodrigo Siqueira <Rodrigo.Siqueira@....com>
Cc:     Brian Starkey <brian.starkey@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Simon Ser <contact@...rsion.fr>,
        Leandro Ribeiro <leandro.ribeiro@...labora.com>,
        Helen Koike <helen.koike@...labora.com>,
        Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH V4 2/3] drm/vkms: Compute CRC without change input data

Hi Rodrigo,

On Mon, 11 May 2020 at 12:55, Rodrigo Siqueira <Rodrigo.Siqueira@....com> wrote:
>
> From: Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
>
> 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.
>
> 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 258e659ecfba..686d25e7b01d 100644
> --- a/drivers/gpu/drm/vkms/vkms_composer.c
> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
> @@ -9,33 +9,40 @@
>
>  #include "vkms_drv.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));
>                 }
>         }
>
IMHO using something like the following makes the code far simpler and clearer.

offset = composer->offset + (y_src * composer->pitch) + (x_src * composer->cpp);

for (i = 0; i < h_src; i++, offset += composer->pitch) {
   for (j = 0; j < w_src; j++, offset += composer->cpp) {
      pixel = get_pixel_from_buffer(vaddr, offset);
      crc = crc32_le(crc, &pixel, sizeof(u32); // cast should not be needed
   }
}

With the bitmap_clear() and related comment moved into get_pixel_from_buffer().

-Emil