Re: [PATCH 2/7] drm/format-helper: Add drm_fb_xrgb8888_to_rgb332()

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Aug 17, 2021 at 02:29:12PM +0200, Noralf Trønnes wrote:
> Add XRGB8888 emulation support for devices that can only do RGB332.
> 
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
> ---
>  drivers/gpu/drm/drm_format_helper.c | 47 +++++++++++++++++++++++++++++
>  include/drm/drm_format_helper.h     |  2 ++
>  2 files changed, 49 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
> index 5231104b1498..53b426da7467 100644
> --- a/drivers/gpu/drm/drm_format_helper.c
> +++ b/drivers/gpu/drm/drm_format_helper.c
> @@ -135,6 +135,53 @@ void drm_fb_swab(void *dst, void *src, struct drm_framebuffer *fb,
>  }
>  EXPORT_SYMBOL(drm_fb_swab);
>  
> +static void drm_fb_xrgb8888_to_rgb332_line(u8 *dbuf, u32 *sbuf, unsigned int pixels)
> +{
> +	unsigned int x;
> +
> +	for (x = 0; x < pixels; x++)
> +		dbuf[x] = ((sbuf[x] & 0x00e00000) >> 16) |

I think for 2/3 bits correct rounding would be useful, not just masking.
i.e. before you shift add 0x00100000 here, and similar below.

Also I just realized we've totally ignored endianess on these, which is
not great, because strictly speaking all the drm_fourcc codes should be
little endian. But I'm also not sure that's worth fixing ...

Either way, lgtm:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> +			  ((sbuf[x] & 0x0000e000) >> 11) |
> +			  ((sbuf[x] & 0x000000c0) >> 6);
> +}
> +
> +/**
> + * drm_fb_xrgb8888_to_rgb332 - Convert XRGB8888 to RGB332 clip buffer
> + * @dst: RGB332 destination buffer
> + * @src: XRGB8888 source buffer
> + * @fb: DRM framebuffer
> + * @clip: Clip rectangle area to copy
> + *
> + * Drivers can use this function for RGB332 devices that don't natively support XRGB8888.
> + *
> + * This function does not apply clipping on dst, i.e. the destination is a small buffer
> + * containing the clip rect only.
> + */
> +void drm_fb_xrgb8888_to_rgb332(void *dst, void *src, struct drm_framebuffer *fb,
> +			       struct drm_rect *clip)
> +{
> +	size_t width = drm_rect_width(clip);
> +	size_t src_len = width * sizeof(u32);
> +	unsigned int y;
> +	void *sbuf;
> +
> +	/* Use a buffer to speed up access on buffers with uncached read mapping (i.e. WC) */
> +	sbuf = kmalloc(src_len, GFP_KERNEL);
> +	if (!sbuf)
> +		return;
> +
> +	src += clip_offset(clip, fb->pitches[0], sizeof(u32));
> +	for (y = 0; y < drm_rect_height(clip); y++) {
> +		memcpy(sbuf, src, src_len);
> +		drm_fb_xrgb8888_to_rgb332_line(dst, sbuf, width);
> +		src += fb->pitches[0];
> +		dst += width;
> +	}
> +
> +	kfree(sbuf);
> +}
> +EXPORT_SYMBOL(drm_fb_xrgb8888_to_rgb332);
> +
>  static void drm_fb_xrgb8888_to_rgb565_line(u16 *dbuf, u32 *sbuf,
>  					   unsigned int pixels,
>  					   bool swab)
> diff --git a/include/drm/drm_format_helper.h b/include/drm/drm_format_helper.h
> index 4e0258a61311..d0809aff5cf8 100644
> --- a/include/drm/drm_format_helper.h
> +++ b/include/drm/drm_format_helper.h
> @@ -16,6 +16,8 @@ void drm_fb_memcpy_dstclip(void __iomem *dst, unsigned int dst_pitch, void *vadd
>  			   struct drm_rect *clip);
>  void drm_fb_swab(void *dst, void *src, struct drm_framebuffer *fb,
>  		 struct drm_rect *clip, bool cached);
> +void drm_fb_xrgb8888_to_rgb332(void *dst, void *vaddr, struct drm_framebuffer *fb,
> +			       struct drm_rect *clip);
>  void drm_fb_xrgb8888_to_rgb565(void *dst, void *vaddr,
>  			       struct drm_framebuffer *fb,
>  			       struct drm_rect *clip, bool swab);
> -- 
> 2.32.0
> 

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