Re: [PATCH v2 1/4] drm/format-helper: Add drm_fb_{xrgb8888,gray8}_to_mono_reversed()

From: Thomas Zimmermann
Date: Fri Feb 04 2022 - 15:35:28 EST

Hi

Am 04.02.22 um 20:31 schrieb Javier Martinez Canillas:
Hello Thomas,

Thanks a lot for your feedback.

On 2/4/22 16:52, Thomas Zimmermann wrote:

[snip]

+static void drm_fb_gray8_to_mono_reversed_line(u8 *dst, const u8 *src, size_t pixels)
+{
+ unsigned int xb, i;
+
+ for (xb = 0; xb < pixels / 8; xb++) {

In practice, all mode widths are multiples of 8 because VGA mandated it.
So it's ok-ish to assume this here. You should probably at least print a
warning somewhere if (pixels % 8 != 0)


Agreed.

After sending the mail, I realized that some code copies only parts of the source around; specifically for damage handling. None of this is aligned to multiple of 8. So the copying could start and end in the middle of bytes. You'd need a pixel-offset value of some sort.

If you don't want to handle this now, maybe at least detect this case and put a warning somewhere.

[snip]

+ * DRM doesn't have native monochrome or grayscale support.
+ * Such drivers can announce the commonly supported XR24 format to userspace
+ * and use drm_fb_xrgb8888_to_gray8() to convert to grayscale and then this
+ * helper function to convert to the native format.
+ */
+void drm_fb_gray8_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *src,
+ const struct drm_rect *clip)

There's a bug here. You want to pass in a drm_framebuffer as fourth
argument.

+{
+
+ size_t height = drm_rect_height(clip);
+ size_t width = drm_rect_width(clip);
+ unsigned int y;
+ const u8 *gray8 = src;
+ u8 *mono = dst;
+
+ if (!dst_pitch)
+ dst_pitch = width;

The dst_pitch is given in bytes. You have to device by 8. Here would be
a good place to warn if (width % 8 != 0).


Ok.
+
+ for (y = 0; y < height; y++) {
+ drm_fb_gray8_to_mono_reversed_line(mono, gray8, dst_pitch);
+ mono += (dst_pitch / 8);

The dst_pitch is already given in bytes.


Yes, I know but for reversed mono we want only 1/8 of the width since we
are converting from 8 bits per pixel greyscale to 1 bit per pixel mono.

Or am I misunderstanding what you meant ?

I mean that if there are 80 pixel on a scanline, the value of dst_pitch is already 10. These pitch values are always given in bytes.

Best regards
Thomas


+ gray8 += dst_pitch;

'gray8 += fb->pitches[0]' would be correct.


Ok.
[snip]

+ */
+void drm_fb_xrgb8888_to_mono_reversed(void *dst, unsigned int dst_pitch, const void *src,
+ const struct drm_framebuffer *fb,
+ const struct drm_rect *clip)
+{
+ if (WARN_ON(fb->format->format != DRM_FORMAT_XRGB8888))
+ return;
+
+ if (!dst_pitch)
+ dst_pitch = drm_rect_width(clip);
+
+ drm_fb_xrgb8888_to_gray8(dst, dst_pitch, src, fb, clip);
+ drm_fb_gray8_to_mono_reversed(dst, dst_pitch, dst, fb, clip);

Converting from dst into dst can give incorrect results. At some point
we probably want to add restrict qualifiers to these pointers, to help
the compiler with optimizing.

A better approach here is to pull the per-line conversion from
drm_fb_xrgb8888_to_gray8() into a separate helper and implement a
line-by-line conversion here. something like this:

drm_fb_xrgb8888_to_mono_reversed()
{
char *tmp = kmalloc(size of a single line of gray8)

for (heigth) {
drm_fb_xrgb8888_to_gray8_line(tmp, ..., src, ...);
drm_fb_gray8_to_mono_reversed(dst, ..., tmp, ...);

src += fb->pitches[0]
dst += dst_pitch;
}

kfree(tmp);
}


I see. Yes, that sounds a much better approach. I'll change it in v3.
Best regards,

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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