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

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Date:   Tue, 29 Jan 2019 18:50:20 -0700
From:   shuah <shuah@...nel.org>
To:     Hans Verkuil <hverkuil@...all.nl>, mchehab@...nel.org,
        perex@...ex.cz, tiwai@...e.com
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org, shuah <shuah@...nel.org>
Subject: Re: [PATCH v10 0/4] Media Device Allocator API

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

> 
> BTW, I think there was also inconsistent behavior in the order of streaming
> audio and video: if I stream video first, then I can stream audio afterwards.
> But if I stream audio first, then (if I remember correctly) I can't start
> video streaming.
> 

I will run some tests tomorrow and see what I find. Which video apps are
you running for these tests?

thanks,
-- Shuah