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Date:   Thu, 21 Jul 2022 18:16:09 +0200
From:   Sebastian Fricke <sebastian.fricke@collabora.com>
To:     Nicolas Dufresne <nicolas.dufresne@collabora.com>
Cc:     Robin Murphy <robin.murphy@arm.com>, linux-media@vger.kernel.org,
        jernej.skrabec@gmail.com, knaerzche@gmail.com,
        kernel@collabora.com, bob.beckett@collabora.com,
        ezequiel@vanguardiasur.com.ar, mchehab@kernel.org,
        gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        linux-rockchip@lists.infradead.org, linux-staging@lists.linux.dev,
        Yury Norov <yury.norov@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH 0/6] RkVDEC HEVC driver
Message-ID: <20220721161609.zyhhg3smbusl6koz@basti-XPS-13-9310>
References: <20220713162449.133738-1-sebastian.fricke@collabora.com>
 <7be996ee-9977-129b-08e2-12bde7ac9cd7@arm.com>
 <f05896551f8545af3c7352a6bd38248e038b61d2.camel@collabora.com>
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Hey Nicolas & Robin,

Thanks for the feedback.

On 15.07.2022 11:36, Nicolas Dufresne wrote:
>Le vendredi 15 juillet 2022 à 12:04 +0100, Robin Murphy a écrit :
>> On 2022-07-13 17:24, Sebastian Fricke wrote:
>> > Implement the HEVC codec variation for the RkVDEC driver. Currently only
>> > the RK3399 is supported, but it is possible to enable the RK3288 as it
>> > also supports this codec.
>> >
>> > Based on top of the media tree @ef7fcbbb9eabbe86d2287484bf366dd1821cc6b8
>> > and the HEVC uABI MR by Benjamin Gaignard.
>> > (https://patchwork.linuxtv.org/project/linux-media/list/?series=8360)
>> >
>> > Tested with the GStreamer V4L2 HEVC plugin:
>> > (https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1079)
>> >
>> > Current Fluster score:
>> > `Ran 131/147 tests successfully               in 278.568 secs`
>> > with
>> > `python3 fluster.py run -d GStreamer-H.265-V4L2SL-Gst1.0 -ts JCT-VC-HEVC_V1 -j1`
>> >
>> > failed conformance tests:
>> > - DBLK_D_VIXS_2 (Success on Hantro G2)
>> > - DSLICE_A_HHI_5 (Success on Hantro G2)
>> > - EXT_A_ericsson_4 (Success on Hantro G2)
>> > - PICSIZE_A_Bossen_1 (Hardware limitation)
>> > - PICSIZE_B_Bossen_1 (Hardware limitation)
>> > - PICSIZE_C_Bossen_1 (Hardware limitation)
>> > - PICSIZE_D_Bossen_1 (Hardware limitation)
>> > - PPS_A_qualcomm_7 (Success on Hantro G2)
>> > - SAODBLK_A_MainConcept_4 (Success on Hantro G2)
>> > - SAODBLK_B_MainConcept_4 (Success on Hantro G2)
>> > - SLIST_B_Sony_9 (Success on Hantro G2)
>> > - SLIST_D_Sony_9 (Success on Hantro G2)
>> > - TSUNEQBD_A_MAIN10_Technicolor_2 (Success on Hantro G2)
>> > - VPSSPSPPS_A_MainConcept_1 (Success on Hantro G2)
>> > - WPP_D_ericsson_MAIN10_2 (Fail on Hantro G2)
>> > - WPP_D_ericsson_MAIN_2 (Fail on Hantro G2)
>> >
>> > Not tested with FFMpeg so far.
>> >
>> > Known issues:
>> > - Unable to reliably decode multiple videos concurrently
>> > - The SAODBLK_* tests timeout if the timeout time in fluster is lower than 120
>> > - Currently the uv_virstride is calculated in a manner that is hardcoded
>> > for the two available formats NV12 and NV15. (@config_registers)
>> >
>> > Notable design decisions:
>> > - I opted for a bitfield to represent the PPS memory blob as it is the
>> > perfect tool for that job. It describes the memory layout with any
>> > additional required documentation, is easy to read and a native language
>> > tool for that job
>>
>> Can I point out how terrible an idea this is? The C language gives
>> virtually zero guarantee about how bitfields are actually represented in
>> memory. Platform ABIs (e.g. [1]) might nail things down a bit more, but
>> different platforms are free to make completely different choices so
>> portability still goes out the window. Even for a single platform,
>> different compilers (or at worst even different version of one compiler)
>> can still make incompatible choices e.g. WRT alignment of packed
>> members. Even if you narrow the scope as far as a specific version of
>> AArch64 GCC, I think this is still totally broken for big-endian.
>>
>> The fact that you've had to use nonsensical types to trick a compiler
>> into meeting your expectations should already be a clue to how fragile
>> this is in general.
>>
>> > - The RPS memory blob is created using a bitmap implementation, which
>> > uses a common Kernel API to avoid reinventing the wheel and to keep the
>> > code clean.
>>
>> Similarly, Linux bitmaps are designed for use as, well, bitmaps. Abusing
>> them as a data interchange format for bit-aligned numerical values is
>> far from "clean" semantically. And I'm pretty sure it's also broken for
>> big-endian.
>>
>> This kind of stuff may be standard practice in embedded development
>> where you're targeting a specific MCU with a specific toolchain, but I
>> don't believe it's suitable for upstream Linux. It would take pretty
>> much the same number of lines to use GENMASK definitions and bitfield.h
>> helpers to pack values into words which can then be written to memory in
>> a guaranteed format and endianness (certainly for the PPS; for the RPS
>> it may well end up a bit longer, but would be self-documenting and
>> certainly more readable than those loops). It mostly just means that for
>> any field which crosses a word boundary you'll end up with 2 definitions
>> and 2 assignments, which is hardly a problem (and in some ways more
>> honest about what's actually going on).
>
>Thanks for the feedback, in multimedia (unlike register programming), we don't
>really consider bitstreams as bitmap or bitfield. What we do really expect is to
>use bit writer helpers (and sometimes a bit reader though we try and avoid the
>second one in the  kernel). Its more of less a cursor (a bit position) into a
>memory that advance while writing. A bit writer should help protect against
>overflow too.
>
>When writing lets say a chain of 8 bits from a char, a proper helper is expected
>to be very explicit on the ordering (write_u8_le/be or something better worded).
>I would rather like to see all these blobs written this way personally then
>having a cleared buffer and writing using bit offsets.
>
>Perhaps I may suggest to start with implementing just that inside this driver?
>It isn't very hard, and then the implementation can be reduced later and shared
>later, with whatever exists without deviating from the intent of the existing
>API ? I do believe that having this in linux-media can be useful in the future.
>We will notably need to extend such a helper with multimedia specific coding
>technique (golomb, boolean coding, etc.) for use in stateless encoder drivers.

I currently design a general bit-writer API to handle the mentioned
issues correctly. I'll post it as part of V2, due to my current workload
this will happen in 3 weeks at the earliest.

>
>Nicolas
>
>>
>> Thanks,
>> Robin.

Greetings,
Sebastian

>>
>> [1]
>> https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#bit-fields
>>
>> > - I deliberatly opted against the macro solution used in H264, which
>> > declares Macros in mid function and declares the fields of the memory
>> > blob as macros as well. And I would be glad to refactor the H264 code if
>> > desired by the maintainer to use common Kernel APIs and native language
>> > elements.
>> > - The giant static array of cabac values is moved to a separate c file,
>> > I did so because a separate .h file would be incorrect as it doesn't
>> > expose anything of any value for any other file than the rkvdec-hevc.c
>> > file. Other options were:
>> >    - Calculating the values instead of storing the results (doesn't seem
>> >    to be worth it)
>> >    - Supply them via firmware (Adding firmware makes the whole software
>> >    way more complicated and the usage of the driver less obvious)
>> >
>> > Ignored Checkpatch warnings (as it fits to the current style of the file):
>> > ```
>> > WARNING: line length of 162 exceeds 100 columns
>> > #115: FILE: drivers/media/v4l2-core/v4l2-common.c:265:
>> > +               { .format = V4L2_PIX_FMT_NV15,    .pixel_enc = V4L2_PIXEL_ENC_YUV, .mem_planes = 1, .comp_planes = 2, .bpp = { 5, 5, 0, 0 }, .hdiv = 2, .vdiv = 2,
>> >
>> > ERROR: trailing statements should be on next line
>> > #128: FILE: drivers/media/v4l2-core/v4l2-ioctl.c:1305:
>> > +       case V4L2_PIX_FMT_NV15:         descr = "10-bit Y/CbCr 4:2:0 (Packed)"; break;
>> > ```
>> >
>> > v4l2-compliance test:
>> > ```
>> > Total for rkvdec device /dev/video3: 46, Succeeded: 46, Failed: 0, Warnings: 0
>> > ```
>> >
>> > kselftest module run for the bitmap changes:
>> > ```
>> > $ sudo insmod /usr/lib/modules/5.19.0-rc3-finalseries/kernel/lib/test_bitmap.ko
>> > [   71.751716] test_bitmap: parselist: 14: input is '0-2047:128/256' OK, Time: 1750
>> > [   71.751787] test_bitmap: bitmap_print_to_pagebuf: input is '0-32767
>> > [   71.751787] ', Time: 6708
>> > [   71.760373] test_bitmap: set_value: 6/6 tests correct
>> > ```
>> >
>> > Jonas Karlman (2):
>> >    media: v4l2: Add NV15 pixel format
>> >    media: v4l2-common: Add helpers to calculate bytesperline and
>> >      sizeimage
>> >
>> > Sebastian Fricke (4):
>> >    bitops: bitmap helper to set variable length values
>> >    staging: media: rkvdec: Add valid pixel format check
>> >    staging: media: rkvdec: Enable S_CTRL IOCTL
>> >    staging: media: rkvdec: Add HEVC backend
>> >
>> >   .../media/v4l/pixfmt-yuv-planar.rst           |   53 +
>> >   drivers/media/v4l2-core/v4l2-common.c         |   79 +-
>> >   drivers/media/v4l2-core/v4l2-ioctl.c          |    1 +
>> >   drivers/staging/media/rkvdec/Makefile         |    2 +-
>> >   drivers/staging/media/rkvdec/TODO             |   22 +-
>> >   .../staging/media/rkvdec/rkvdec-hevc-data.c   | 1844 +++++++++++++++++
>> >   drivers/staging/media/rkvdec/rkvdec-hevc.c    |  859 ++++++++
>> >   drivers/staging/media/rkvdec/rkvdec-regs.h    |    1 +
>> >   drivers/staging/media/rkvdec/rkvdec.c         |  182 +-
>> >   drivers/staging/media/rkvdec/rkvdec.h         |    3 +
>> >   include/linux/bitmap.h                        |   39 +
>> >   include/uapi/linux/videodev2.h                |    1 +
>> >   lib/test_bitmap.c                             |   47 +
>> >   13 files changed, 3066 insertions(+), 67 deletions(-)
>> >   create mode 100644 drivers/staging/media/rkvdec/rkvdec-hevc-data.c
>> >   create mode 100644 drivers/staging/media/rkvdec/rkvdec-hevc.c
>> >
>>
>