Re: [PATCH v10 0/4] Media Device Allocator API

From: shuah
Date: Tue Feb 05 2019 - 13:11:12 EST

On 2/1/19 2:21 AM, Hans Verkuil wrote:
On 2/1/19 1:46 AM, shuah wrote:
Hi Hans,

On 1/30/19 12:42 AM, Hans Verkuil wrote:
On 1/30/19 2:50 AM, shuah wrote:
On 1/29/19 2:43 AM, Hans Verkuil wrote:
On 1/29/19 12:48 AM, shuah wrote:
Hi Hans,

On 1/28/19 5:03 AM, Hans Verkuil wrote:
Hi Shuah,

On 1/24/19 9:32 PM, Shuah Khan wrote:
Media Device Allocator API to allows multiple drivers share a media device.
This API solves a very common use-case for media devices where one physical
device (an USB stick) provides both audio and video. When such media device
exposes a standard USB Audio class, a proprietary Video class, two or more
independent drivers will share a single physical USB bridge. In such cases,
it is necessary to coordinate access to the shared resource.

Using this API, drivers can allocate a media device with the shared struct
device as the key. Once the media device is allocated by a driver, other
drivers can get a reference to it. The media device is released when all
the references are released.

- This patch series is tested on 5.0-rc3 and addresses comments on
ÂÂÂÂ v9 series from Hans Verkuil.
- v9 was tested on 4.20-rc6.
- Tested sharing resources with kaffeine, vlc, xawtv, tvtime, and
ÂÂÂÂ arecord. When analog is streaming, digital and audio user-space
ÂÂÂÂ applications detect that the tuner is busy and exit. When digital
ÂÂÂÂ is streaming, analog and audio applications detect that the tuner is
ÂÂÂÂ busy and exit. When arecord is owns the tuner, digital and analog
ÂÂÂÂ detect that the tuner is busy and exit.

I've been doing some testing with my au0828, and I am confused about one
thing, probably because it has been too long ago since I last looked into
this in detail:


Great.

Why can't I change the tuner frequency if arecord (and only arecord) is
streaming audio? If arecord is streaming, then it is recording the audio
from the analog TV tuner, right? So changing the analog TV frequency
should be fine.


Changing analog TV frequency would be s_frequency. The way it works is
any s_* calls would require holding the pipeline. In Analog TV case, it
would mean holding both audio and video pipelines for any changes
including TV.

As I recall, we discussed this design and the decision was to make all
s_* calls interfaces to hold the tuner. A special exception is g_tuner
in case of au0828. au0828 initializes the tuner from s_* interfaces and
its g_tuner interfaces. Allowing s_frequency to proceed will disrupt the
arecord audio stream.

Query (q_*) works just fine without holding the pipeline. I limited the
analog holds to just the ones that are required. The current set is
required to avoid audio stream disruptions.

So I am not sure about that ('avoid audio stream disruptions'): if I
stream video AND use arecord, then I can just set the frequency while
streaming. Doesn't that interrupt audio as well? And are you sure changing
the tuner frequency actually disrupts audio? And if audio is disrupted,
are we talking about a glitch or is audio permanently disrupted?

I think it is a glitch. I will run some tests and let you know.

That's basically the inconsistent behavior I noticed: just running arecord
will prevent me from changing the frequency, but if I run arecord and stream
video, then it is suddenly OK to change the frequency.

How are you changing frequency? I want to duplicate what you are doing.

v4l2-ctl -f <freq>

I am not seeing the inconsistent behavior. Here are my results.

1. Started acecord and while it is running:

arecord -M -D plughw:2,0 -c2Â -f S16_LE -t wav foo.wav
Recording WAVE 'foo.wav' : Signed 16 bit Little Endian, Rate 8000 Hz, Stereo

2. Ran v4l2-ctl -f as follows:

v4l2-ctl -f 700
VIDIOC_G_TUNER: failed: Device or resource busy
VIDIOC_S_FREQUENCY: failed: Device or resource busy

Based on the current implementation, it failed with resource
busy as expected.

3. Started v4l2-ctl as follows:

Âv4l2-ctl --stream-mmap --stream-count=100 -d /dev/video0
VIDIOC_STREAMON: failed: Device or resource busy

Why is this? You have one analog tuner and it delivers independent audio
and video streams. I should be able to start/stop audio and video independently.

And as mentioned above, if I use v4l2-ctl for video streaming, then start arecord,
then that works fine. And I can change the tuner frequency while both are streaming.

But doing this the other way around (first start arecord, then v4l2-ctl) then that
doesn't work.

It makes no sense.

Note that v4l2-ctl does not open audio, it solely deals with video. qv4l2 will
open both audio and video, but for testing audio and video independently you
need to use arecord/v4l2-ctl.

In any case, my understanding of how this should work is that both arecord and
v4l2-ctl should attempt to lock the analog tuner resource, but they can share it.

If DVB is using the tuner, then both arecord and v4l2-ctl should fail with -EBUSY.
Same if either of arecord/v4l2-ctl is running, then DVB should fail with -EBUSY.

BTW, I won't be able to test anything myself until Feb 9th since I'm abroad and
don't have access to my au0828.


I am also on a business trip at the moment and won't have access to my
au0828 until Feb 8th.

I think I understand your concern and the change to get the desired behavior of allowing tuner to be shared by arecord and v4l2-ctl is
going to in the code that is already in the mainline which is the
au0828_enable_source().

If you would like to wait to pull this series for the problem to be fixed, I can send that change in. It will be another patch added to
the series.

Or if you pull this and I can send fix on top. Let me know your preference.

thanks,
-- Shuah