[Intel-gfx] [PATCH 1/8] drm/i915: Implement .get_format_info() hook for CCS

[Daniel Stone]

Hi Vidya,

On 7 June 2017 at 12:40, Vidya Srinivas <vidya.srinivas at intel.com> wrote:
> +static const struct drm_format_info ccs_formats[] = {
> +       { .format = DRM_FORMAT_XRGB8888, .depth = 24, .num_planes = 2, .cpp = { 4, 1, }, .hsub = 16, .vsub = 8, },
> +       { .format = DRM_FORMAT_XBGR8888, .depth = 24, .num_planes = 2, .cpp = { 4, 1, }, .hsub = 16, .vsub = 8, },
> +       { .format = DRM_FORMAT_ARGB8888, .depth = 32, .num_planes = 2, .cpp = { 4, 1, }, .hsub = 16, .vsub = 8, },
> +       { .format = DRM_FORMAT_ABGR8888, .depth = 32, .num_planes = 2, .cpp = { 4, 1, }, .hsub = 16, .vsub = 8, },
> +};

I notice that here hsub/vsub are declared as 16x8. In Ville's tree
which I pulled my submission from, this is 8x16, which aligns with
this comment (missing from your submission):
/*
 * 1 byte of CCS actually corresponds to 16x8 pixels on the main
 * surface, and the memory layout for the CCS tile us 64x64 bytes.
 * But since we're pretending the CCS tile is 128 bytes wide we
 * adjust hsub/vsub here accordingly to 8x16 so that the
 * bytes<->x/y conversions come out correct.
 */

If the hsub/vsub is inverted to match the comment, trying to add a
3200x1800 (for example) framebuffer will fail, because (1800 % 16) !=
0. This is true even if the allocation is correct (i.e. the buffer
contains an even number of tiles for both width and height). Generic
userspace cannot know that it should try to create a larger
framebuffer (in this case 3200x1808) and only show a smaller region of
that framebuffer.

This is the reason for the halign/valign patch mentioned earlier, as
well as the additional part of the comment which was also present in
my submission:
/*
 * We don't require any
 * CCS block size alignment of the fb under the assumption that the
 * hardware will handle things correctly of only a single pixel
 * gets touched. The compression should be lossless so any garbage
 * pixels as part of the same block shouldn't cause visual artifacts.
 */

Cheers,
Daniel