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Subject: Re: [PATCH 1/5] media: videodev2: add Compressed Framebuffer pixel
 formats
To:     Nicolas Dufresne <nicolas@ndufresne.ca>, hverkuil-cisco@xs4all.nl
Cc:     linux-media@vger.kernel.org, linux-amlogic@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Maxime Jourdan <mjourdan@baylibre.com>
References: <20200604135317.9235-1-narmstrong@baylibre.com>
 <20200604135317.9235-2-narmstrong@baylibre.com>
 <02aa06fd8397b77c9a75d3a8399cb55d3b4d39c1.camel@ndufresne.ca>
From:   Neil Armstrong <narmstrong@baylibre.com>
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Organization: Baylibre
Message-ID: <4d22ff40-11ac-c77a-564d-af9a678f23af@baylibre.com>
Date:   Mon, 8 Jun 2020 10:16:50 +0200
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Nicolas,

On 05/06/2020 17:35, Nicolas Dufresne wrote:
> Le jeudi 04 juin 2020 à 15:53 +0200, Neil Armstrong a écrit :
>> From: Maxime Jourdan <mjourdan@baylibre.com>
>>
>> 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@baylibre.com>
>> Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
>> ---
>>  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;

Seems we are totally on-par with the following :-)

> 
> 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.

Yes, and this is exactly my goal here, and matches the Amlogic Framebuffer as
described in patch 4. The modifier patchset is at [1].

> 
> 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). 

It's even more complex, the modifier depends on the SoC revision, so we can
have up to6 different unique pixel format instead of 2 with a variable
modifier.

> 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.

Yes, but until the migration of drm_fourcc and v4l2 fourcc into a common one
is decided, I'm stuck and this is the only intermediate solution I found.

We have a working solution with Boris's patchset with ext_fmt passing the
modifier to user-space.

but anyway, since the goal is to merge the fourcc between DRM & V4L2, these YUV420_*BIT
will still be needed if we pass the modifier with an extended format struct.

> 
>>  		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 */
> 

[1] https://patchwork.freedesktop.org/series/73722/#rev7