[PATCH 0/4] media: platform: Add Aspeed Video Engine driver

Eddie James eajames at linux.vnet.ibm.com
Sat Sep 15 01:07:37 AEST 2018

On 09/14/2018 01:56 AM, Hans Verkuil wrote:
> On 09/13/2018 09:11 PM, Eddie James wrote:
>> On 09/03/2018 06:57 AM, Hans Verkuil wrote:
>>> Hi Eddie,
>>> Thank you for your work on this. Interesting to see support for this SoC :-)
>>> On 08/29/2018 11:09 PM, Eddie James wrote:
>>>> The Video Engine (VE) embedded in the Aspeed AST2400 and AST2500 SOCs
>>>> can capture and compress video data from digital or analog sources. With
>>>> the Aspeed chip acting as a service processor, the Video Engine can
>>>> capture the host processor graphics output.
>>>> This series adds a V4L2 driver for the VE, providing a read() interface
>>>> only. The driver triggers the hardware to capture the host graphics output
>>>> and compress it to JPEG format.
>>>> Testing on an AST2500 determined that the videobuf/streaming/mmap interface
>>>> was significantly slower than the simple read() interface, so I have not
>>>> included the streaming part.
>>> Do you know why? It should be equal or faster, not slower.
>> Yes, it seems to be an issue with the timing of the video engine
>> interrupts compared with how a normal v4l2 application queues buffers.
>> With the simple read() application, the driver can swap between DMA
>> buffers freely and get a frame ahead. With the streaming buffers, I
>> found the driver ran through the queue quite quickly, but then, once
>> userspace queues again, we had to wait for the next frame, as I couldn't
>> get a frame ahead since no buffers were available during that time
>> period. This could possibly be solved with more buffers but this gets to
>> require a lot of memory, since each buffer is allocated for the full
>> frame size even though we only fill a fraction of it with JPEG data...
> For stream I/O you usually need 3 buffers: one is being DMAed, one is
> the next queued up frame for the DMA and the third is being processed
> in userspace. If userspace doesn't process buffers fast enough, then
> the driver will need to capture into the same buffer over and over again
> until userspace finally queues another buffer.
> What I don't understand here is what the frame rate is. Is the capture
> framerate the same as the host processor graphics output? Or is it
> unrelated to that?

Yes, the frame rate is the capture frame rate, unrelated to the host 
graphics output frame rate. It's really just a bandwidth-saving measure.

> The problem with read() is that 1) it requires copying the video data, and
> 2) you cannot use dmabuf for zero-copying pipelines. Whether or not 2 is
> needed depends on your hardware.
>>> I reviewed about half of the driver, but then I stopped since there were too
>>> many things missing.
>>> First of all, you need to test your driver with v4l2-compliance (available here:
>>> https://git.linuxtv.org/v4l-utils.git/). Always compile from the git repo since
>>> the versions from distros tend to be too old.
>>> Just run 'v4l2-compliance -d /dev/videoX' and fix all issues. Then run
>>> 'v4l2-compliance -s -d /dev/videoX' to test streaming.
>>> This utility checks if the driver follows the V4L2 API correctly, implements
>>> all ioctls that it should and fills in all the fields that it should.
>>> Please add the output of 'v4l2-compliance -s' to future versions of this patch
>>> series: I don't accept V4L2 drivers without a clean report of this utility.
>> Sure thing. Thanks for the guidance.
>>> If you have any questions, then mail me or (usually quicker) ask on the #v4l
>>> freenode irc channel (I'm in the CET timezone).
>>> One thing that needs more explanation: from what I could tell from the driver
>>> the VIDIOC_G_FMT ioctl returns the detected format instead of the current
>>> format. This is wrong. Instead you should implement the VIDIOC_*_DV_TIMINGS
>>> ioctls and the V4L2_EVENT_SOURCE_CHANGE event.
>>> The normal sequence is that userspace queries the current timings with
>>> VIDIOC_QUERY_DV_TIMINGS, if it finds valid timings, then it sets these
>>> timings with _S_DV_TIMINGS. Now it can call G/S_FMT. If the timings
>>> change, then the driver should detect that and send a V4L2_EVENT_SOURCE_CHANGE
>>> event.
>> OK I see. I ended up simplifying this part anyway since it's not
>> possible to change the video size from the driver. I don't think there
>> is a need for VIDIOC_QUERY_DV_TIMINGS now, but feel free to review.
> You are capturing the host graphics output, right? So you need to know
> the resolution, timings, etc. of that output? What if the host changes
> resolution? That's something you need to know, or am I missing something?

Yes, that's why I had those extra lines in set_format, so that if the 
set format resolution matches the actual frame size, then it can 
change... I guess I'm probably mis-using the API. Will look into the 
source event change and timings calls.

> This is obviously a somewhat different environment than what I am used to,
> so bear with me if I ask stupid questions...

No problem!

> Regards,
> 	Hans
>> Thanks again,
>> Eddie
>>> When the application receives this event it can take action, such as
>>> increasing the size of the buffer for the jpeg data that it reads into.
>>> The reason for this sequence of events is that you can't just change the
>>> format/resolution mid-stream without giving userspace the chance to
>>> reconfigure.
>>> Regards,
>>> 	Hans
>>>> It's also possible to use an automatic mode for the VE such that
>>>> re-triggering the HW every frame isn't necessary. However this wasn't
>>>> reliable on the AST2400, and probably used more CPU anyway due to excessive
>>>> interrupts. It was approximately 15% faster.
>>>> The series also adds the necessary parent clock definitions to the Aspeed
>>>> clock driver, with both a mux and clock divider.
>>>> Eddie James (4):
>>>>     clock: aspeed: Add VIDEO reset index definition
>>>>     clock: aspeed: Setup video engine clocking
>>>>     dt-bindings: media: Add Aspeed Video Engine binding documentation
>>>>     media: platform: Add Aspeed Video Engine driver
>>>>    .../devicetree/bindings/media/aspeed-video.txt     |   23 +
>>>>    drivers/clk/clk-aspeed.c                           |   41 +-
>>>>    drivers/media/platform/Kconfig                     |    8 +
>>>>    drivers/media/platform/Makefile                    |    1 +
>>>>    drivers/media/platform/aspeed-video.c              | 1307 ++++++++++++++++++++
>>>>    include/dt-bindings/clock/aspeed-clock.h           |    1 +
>>>>    6 files changed, 1379 insertions(+), 2 deletions(-)
>>>>    create mode 100644 Documentation/devicetree/bindings/media/aspeed-video.txt
>>>>    create mode 100644 drivers/media/platform/aspeed-video.c

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