Re: [PATCH 1/5] media: videodev2: add Compressed Framebuffer pixel formats

[Nicolas Dufresne]

Le vendredi 05 juin 2020 Ã 11:35 -0400, Nicolas Dufresne a Ãcrit :
> Le jeudi 04 juin 2020 Ã 15:53 +0200, Neil Armstrong a Ãcrit :
> > From: Maxime Jourdan <mjourdan@xxxxxxxxxxxx>
> > 
> > Add two generic Compressed Framebuffer pixel formats to be used
> > with a modifier when imported back in another subsystem like DRM/KMS.
> > 
> > These pixel formats represents generic 8bits and 10bits compressed buffers
> > with a vendor specific layout.
> > 
> > These are aligned with the DRM_FORMAT_YUV420_8BIT and DRM_FORMAT_YUV420_10BIT
> > used to describe the underlying compressed buffers used for ARM Framebuffer
> > Compression. In the Amlogic case, the compression is different but the
> > underlying buffer components is the same.
> > 
> > Signed-off-by: Maxime Jourdan <mjourdan@xxxxxxxxxxxx>
> > Signed-off-by: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
> > ---
> >  drivers/media/v4l2-core/v4l2-ioctl.c | 2 ++
> >  include/uapi/linux/videodev2.h       | 9 +++++++++
> >  2 files changed, 11 insertions(+)
> > 
> > diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
> > index 2322f08a98be..8f14adfd5bc5 100644
> > --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> > +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> > @@ -1447,6 +1447,8 @@ static void v4l_fill_fmtdesc(struct v4l2_fmtdesc *fmt)
> >  		case V4L2_PIX_FMT_S5C_UYVY_JPG:	descr = "S5C73MX interleaved UYVY/JPEG"; break;
> >  		case V4L2_PIX_FMT_MT21C:	descr = "Mediatek Compressed Format"; break;
> >  		case V4L2_PIX_FMT_SUNXI_TILED_NV12: descr = "Sunxi Tiled NV12 Format"; break;
> > +		case V4L2_PIX_FMT_YUV420_8BIT:	descr = "Compressed YUV 4:2:0 8-bit Format"; break;
> > +		case V4L2_PIX_FMT_YUV420_10BIT:	descr = "Compressed YUV 4:2:0 10-bit Format"; break;
> 
> When I read the DRM documentation [0], I'm reading that YUV420_8BIT
> definition matches V4L2_PIX_FMT_YVU420 and V4L2_PIX_FMT_YVU420M fully.
> In fact, on DRM side, to represent that format you want to expose here,
> they will strictly combine this generic format (documented un-
> compressed) with a modifier generated with the macro
> DRM_FORMAT_MOD_ARM_AFBC(*). And only the combination represent a unique
> and share-able format.

[0] https://www.kernel.org/doc/html/v5.5-rc1/gpu/afbc.html

> 
> In absence of modifier in V4L2 API, this compressed format should be
> named accordingly to the compressed algorithm used (something like
> FMT_YUV420_8BIT_AML_FBC). So I believe these format name cannot be
> copied as-is like this, as they can only introduce more ambiguity in
> the already quite hard to follow naming of pixel formats. In fact, it
> is very common to see multiple different vendor compressions on the
> same SoC, so I don't really believe a "generic" compressed format make
> sense. To site one, the IMX8M, which got Verrisillicon/Vivante DEC400
> on the GPU, and the Hantro G2 compression format. Both will apply to
> NV12 class of format so in DRM they would be NV12 + modifier, and the
> combination forms the unique format. Now, in term of sharing, they must
> be differiented by userspace, as support for compression/decompression
> is heterogeneous (in that case the GPU does not support Hantro G2
> decompression or compression, and the VPU does not support DEC400).
> 
> I'll remind that the modifier implementation has great value and is
> much more scalable then the current V4L2 approach. There has been some
> early proposal for this, maybe it's time to prioritize because this
> list will starts growing with hundred or even thousands or format,
> which is clearly indicated by the increase of modifier generator macro
> on the DRM side.
> 
> >  		default:
> >  			if (fmt->description[0])
> >  				return;
> > diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
> > index c3a1cf1c507f..90b9949acb8a 100644
> > --- a/include/uapi/linux/videodev2.h
> > +++ b/include/uapi/linux/videodev2.h
> > @@ -705,6 +705,15 @@ struct v4l2_pix_format {
> >  #define V4L2_PIX_FMT_FWHT     v4l2_fourcc('F', 'W', 'H', 'T') /* Fast Walsh Hadamard Transform (vicodec) */
> >  #define V4L2_PIX_FMT_FWHT_STATELESS     v4l2_fourcc('S', 'F', 'W', 'H') /* Stateless FWHT (vicodec) */
> >  
> > +/*
> > + * Compressed Luminance+Chrominance meta-formats
> > + * In these formats, the component ordering is specified (Y, followed by U
> > + * then V), but the exact Linear layout is undefined.
> > + * These formats can only be used with a non-Linear modifier.
> > + */
> > +#define V4L2_PIX_FMT_YUV420_8BIT	v4l2_fourcc('Y', 'U', '0', '8') /* 1-plane YUV 4:2:0 8-bit */
> > +#define V4L2_PIX_FMT_YUV420_10BIT	v4l2_fourcc('Y', 'U', '1', '0') /* 1-plane YUV 4:2:0 10-bit */
> > +
> >  /*  Vendor-specific formats   */
> >  #define V4L2_PIX_FMT_CPIA1    v4l2_fourcc('C', 'P', 'I', 'A') /* cpia1 YUV */
> >  #define V4L2_PIX_FMT_WNVA     v4l2_fourcc('W', 'N', 'V', 'A') /* Winnov hw compress */