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Date:   Wed, 7 Oct 2020 12:03:48 +0100
From:   Matteo Franchin <matteo.franchin@arm.com>
To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     Dave Airlie <airlied@linux.ie>, Liviu Dudau <liviu.dudau@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Thomas Zimmermann <tzimmermann@suse.de>, nd <nd@arm.com>
Subject: Re: [PATCH] drm/fourcc: Add AXBXGXRX106106106106 format
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Hi Daniel,

On Wed, Oct 07, 2020 at 12:06:34PM +0200, Daniel Vetter wrote:
> On Wed, Oct 7, 2020 at 11:29 AM Matteo Franchin <matteo.franchin@arm.com> wrote:
> >
> > Add ABGR format with 10-bit components packed in 64-bit per pixel.
> > This format can be used to handle
> > VK_FORMAT_R10X6G10X6B10X6A10X6_UNORM_4PACK16 on little-endian
> > architectures.
> >
> > Signed-off-by: Matteo Franchin <matteo.franchin@arm.com>
> 
> So is this essentially 16 bit, with the lowest 6 bits in each channel
> ignored? What exactly is this used for where the full 16bit format
> doesn't work?

If you are asking why we don't add a 16-bit format, instead (e.g.
DRM_FORMAT_ABGR16161616) the answer is that this is really a 10-bit
format: only the 10 MSBs are preserved. 

We'd be uncomfortable to add support for a 16-bit format and just
ignore the lowest 6 bits.

Thanks,
Matteo


> -Daniel
> 
> > ---
> >  drivers/gpu/drm/drm_fourcc.c  | 1 +
> >  include/uapi/drm/drm_fourcc.h | 7 +++++++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_fourcc.c b/drivers/gpu/drm/drm_fourcc.c
> > index 722c7ebe4e88..bba03fcb016d 100644
> > --- a/drivers/gpu/drm/drm_fourcc.c
> > +++ b/drivers/gpu/drm/drm_fourcc.c
> > @@ -202,6 +202,7 @@ const struct drm_format_info *__drm_format_info(u32 format)
> >                 { .format = DRM_FORMAT_XBGR16161616F,   .depth = 0,  .num_planes = 1, .cpp = { 8, 0, 0 }, .hsub = 1, .vsub = 1 },
> >                 { .format = DRM_FORMAT_ARGB16161616F,   .depth = 0,  .num_planes = 1, .cpp = { 8, 0, 0 }, .hsub = 1, .vsub = 1, .has_alpha = true },
> >                 { .format = DRM_FORMAT_ABGR16161616F,   .depth = 0,  .num_planes = 1, .cpp = { 8, 0, 0 }, .hsub = 1, .vsub = 1, .has_alpha = true },
> > +               { .format = DRM_FORMAT_AXBXGXRX106106106106,    .depth = 0,  .num_planes = 1, .cpp = { 8, 0, 0 }, .hsub = 1, .vsub = 1, .has_alpha = true },
> >                 { .format = DRM_FORMAT_RGB888_A8,       .depth = 32, .num_planes = 2, .cpp = { 3, 1, 0 }, .hsub = 1, .vsub = 1, .has_alpha = true },
> >                 { .format = DRM_FORMAT_BGR888_A8,       .depth = 32, .num_planes = 2, .cpp = { 3, 1, 0 }, .hsub = 1, .vsub = 1, .has_alpha = true },
> >                 { .format = DRM_FORMAT_XRGB8888_A8,     .depth = 32, .num_planes = 2, .cpp = { 4, 1, 0 }, .hsub = 1, .vsub = 1, .has_alpha = true },
> > diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> > index 82f327801267..76eedba52b77 100644
> > --- a/include/uapi/drm/drm_fourcc.h
> > +++ b/include/uapi/drm/drm_fourcc.h
> > @@ -155,6 +155,13 @@ extern "C" {
> >  #define DRM_FORMAT_ARGB16161616F fourcc_code('A', 'R', '4', 'H') /* [63:0] A:R:G:B 16:16:16:16 little endian */
> >  #define DRM_FORMAT_ABGR16161616F fourcc_code('A', 'B', '4', 'H') /* [63:0] A:B:G:R 16:16:16:16 little endian */
> >
> > +/*
> > + * RGBA format with 10-bit components packed in 64-bit per pixel, with 6 bits
> > + * of unused padding per component:
> > + * [63:0] A:x:B:x:G:x:R:x 10:6:10:6:10:6:10:6 little endian
> > + */
> > +#define DRM_FORMAT_AXBXGXRX106106106106 fourcc_code('A', 'B', '1', '0')
> > +
> >  /* packed YCbCr */
> >  #define DRM_FORMAT_YUYV                fourcc_code('Y', 'U', 'Y', 'V') /* [31:0] Cr0:Y1:Cb0:Y0 8:8:8:8 little endian */
> >  #define DRM_FORMAT_YVYU                fourcc_code('Y', 'V', 'Y', 'U') /* [31:0] Cb0:Y1:Cr0:Y0 8:8:8:8 little endian */
> > --
> > 2.17.1
> >
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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