[PATCH 8/8] drm/format-helper: Optimize 32-to-8-bpp conversion

[Jocelyn Falempe]

On 25/03/2025 11:31, Thomas Zimmermann wrote:
> For ease of implementation, existing line-conversion functions
> for 8-bit formats write each pixel individually. Optimize the
> performance by writing mulitple pixels in a single 32-bit store.

With the commit message typo fixed, mulitple => multiple

Reviewed-by: Jocelyn Falempe <jfalempe at redhat.com>
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> ---
>   drivers/gpu/drm/drm_format_helper.c | 23 ++++++++++++++++++++++-
>   1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
> index 66137df85725..73833db28c3c 100644
> --- a/drivers/gpu/drm/drm_format_helper.c
> +++ b/drivers/gpu/drm/drm_format_helper.c
> @@ -250,10 +250,31 @@ static __always_inline void drm_fb_xfrm_line_32to8(void *dbuf, const void *sbuf,
>   						   unsigned int pixels,
>   						   u32 (*xfrm_pixel)(u32))
>   {
> -	u8 *dbuf8 = dbuf;
> +	__le32 *dbuf32 = dbuf;
> +	u8 *dbuf8;
>   	const __le32 *sbuf32 = sbuf;
>   	const __le32 *send32 = sbuf32 + pixels;
>   
> +	/* write 4 pixels at once */
> +	send32 -= pixels & GENMASK(1, 0);
> +	while (sbuf32 < send32) {
> +		u32 pix[4] = {
> +			le32_to_cpup(sbuf32++),
> +			le32_to_cpup(sbuf32++),
> +			le32_to_cpup(sbuf32++),
> +			le32_to_cpup(sbuf32++),
> +		};
> +		/* write output bytes in reverse order for little endianness */
> +		u32 val32 = xfrm_pixel(pix[0]) |
> +			   (xfrm_pixel(pix[1]) << 8) |
> +			   (xfrm_pixel(pix[2]) << 16) |
> +			   (xfrm_pixel(pix[3]) << 24);
> +		*dbuf32++ = cpu_to_le32(val32);
> +	}
> +	send32 += pixels & GENMASK(1, 0);
> +
> +	/* write trailing pixels */
> +	dbuf8 = (u8 __force *)dbuf32;
>   	while (sbuf32 < send32)
>   		*dbuf8++ = xfrm_pixel(le32_to_cpup(sbuf32++));
>   }