linux-kernel - Re: [PATCH 0/2] Use Media Dev Allocator to fix vimc dev lifetime bugs

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Date:   Fri, 14 Jun 2019 17:26:46 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Helen Koike <helen.koike@...labora.com>,
        Hans Verkuil <hverkuil-cisco@...all.nl>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH 0/2] Use Media Dev Allocator to fix vimc dev lifetime bugs

On 6/13/19 7:24 AM, Helen Koike wrote:
> 
> 
> On 6/13/19 2:44 AM, Hans Verkuil wrote:
>> On 5/24/19 5:31 AM, Shuah Khan wrote:
>>> media_device is embedded in struct vimc_device and when vimc is removed
>>> vimc_device and the embedded media_device goes with it, while the active
>>> stream and vimc_capture continue to access it.
>>>
>>> Fix the media_device lifetime problem by changing vimc to create shared
>>> media_device using Media Device Allocator API and vimc_capture getting
>>> a reference to vimc module. With this change, vimc module can be removed
>>> only when the references are gone. vimc can be removed after vimc_capture
>>> is removed.
>>>
>>> Media Device Allocator API supports just USB devices. Enhance it
>>> adding a genetic device allocate interface to support other media
>>> drivers.
>>>
>>> The new interface takes pointer to struct device instead and creates
>>> media device. This interface allows a group of drivers that have a
>>> common root device to share media device resource and ensure media
>>> device doesn't get deleted as long as one of the drivers holds its
>>> reference.
>>>
>>> The new interface has been tested with vimc component driver to fix
>>> panics when vimc module is removed while streaming is in progress.
>>
>> Helen, can you review this series? I'm not sure this is the right approach
>> for a driver like vimc, and even if it is, then it is odd that vimc-capture
>> is the only vimc module that's handled here.
> 
> Hi Hans,
> 
> Yes, I can take a look. Sorry, I've been a bit busy these days but I'll
> try to take a look at this patch series (and the others) asap.
> 
> Helen
> 
>>
>> My gut feeling is that this should be handled inside vimc directly and not
>> using the media-dev-allocator.
>>

Hi Hans and Helen,

I explored fixing the problem within vimc before I went down the path to
use Media Device Allocator API. I do think that it is cleaner to go this
way and easier to maintain.

vimc is a group pf component drivers that rely on the media device vimc
in vimc and falls into the use-case Media Device Allocator API is added
to address. The release and life-time management happens without vimc
component drivers being changed other than using the API to get and put
media device reference.

thanks,
-- Shuah