On Thu, Aug 11, 2022 at 09:59:39AM +0200, Geert Uytterhoeven wrote:

Hi Daniel,

On Wed, Aug 10, 2022 at 5:59 PM Daniel Vetter [off-list ref] wrote:

quoted

On Fri, Jul 08, 2022 at 08:20:46PM +0200, Geert Uytterhoeven wrote:

quoted

Add a helper to retrieve the actual number of bits per pixel for a
plane, taking into account the number of characters and pixels per
block for tiled formats.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>

quoted

quoted

--- a/drivers/gpu/drm/drm_fourcc.c
+++ b/drivers/gpu/drm/drm_fourcc.c

@@ -370,6 +370,25 @@ unsigned int drm_format_info_block_height(const struct drm_format_info *info,
 }
 EXPORT_SYMBOL(drm_format_info_block_height);

+/**
+ * drm_format_info_bpp - number of bits per pixel
+ * @info: pixel format info
+ * @plane: plane index
+ *
+ * Returns:
+ * The actual number of bits per pixel, depending on the plane index.
+ */
+unsigned int drm_format_info_bpp(const struct drm_format_info *info, int plane)
+{
+     if (!info || plane < 0 || plane >= info->num_planes)
+             return 0;
+
+     return info->char_per_block[plane] * 8 /
+            (drm_format_info_block_width(info, plane) *
+             drm_format_info_block_height(info, plane));

Do we really needs this for blocky formats where this is potentially
ill-defined? I think if there's no need then this should also return 0
when block_width/height != 1, it doesn't make much sense to compute bpp
when it's not really bits per _pixel_.

Yes, we do need this.  For low-color formats, the number of bits
per pixel is less than eight, and block_width is larger than one.
That is actually the point of this patch.

Hm right, I didn't realize that this is how we have to describe the
formats with less than 8 bpp.

I think we can include them easily with a check for char_per_block == 1
and then making sure that the division does not have a reminder (just in
case someone does something really funny, it could e.g. be a 332 layout or
something like that for 3 pixels).

quoted

Minimally this needs to check whether the division actually makes sense or
whether there's a reminder, and if there's  reminder, then fail. But that
feels like a bad hack and I think we should avoid it if it's not
absolutely necessary.

Looking at drivers/gpu/drm/drm_fourcc.c, the only supported format
where there can be a remainder is P030, which has 2 spare bits per
32-bit word, and thus is special anyway.
Still, 4 * 8 / 3 = 10, so you get the correct numbers of bits for
the first plane.  For the second plane, you get 8 * 8 / 3 = 21,
but as .is_yuv = true, you have to divide that result by two again,
so you get 10 again.

Yeah I don't think we should describe these with bpp or cpp or anything
like that. bpp < 8 makes sense since that's how this has been done since
decades, but trying to extend these to funny new formats is a bad idea.
This is also why cpp and depth refuse to compute these (or at least
should).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch