Re: Media Controller - graph_mutex

[Shuah Khan <shuahkh@osg.samsung.com>]

On 07/14/2015 08:06 AM, Sakari Ailus wrote:
> Hi Shuah,
> 
> Shuah Khan wrote:
>> All,
>>
>> ALSA has to make media_entity_pipeline_start() call in irq
>> path. I am seeing warnings that the graph_mutex is unsafe irq
>> lock as expected. We have to update MC start/stop pipeline
>> to be irq safe for ALSA. Maybe there are other MC interfaces
>> that need to be irq safe, but I haven't seen any problems with
>> my limited testing.
>>
>> So as per options, graph_mutex could be changed to a spinlock.
>> It looks like drivers hold this lock and it isn't abstracted to
>> MC API. Unfortunate, this would require changes to drivers that
>> directly hold the lock for graph walks if this mutex is changed
>> to spinlock.
>>
>> e.g: drivers/media/platform/exynos4-is/fimc-isp-video.c
>>
>> Changes aren't complex, just that the scope isn't limited
>> to MC API.
>>
>> Other ideas??
> 
> Do you have (preliminary?) patches and / or more information? I'd like
> to understand why would ALSA need this.
> 
> The graph_mutex is currently also taken when the link state is modified,
> and the callbacks to the drivers may involve power state changes and
> device initialisation. On some busses such as I2C these are blocking
> operations.
> 

I am getting close to sending the RFC patches for review. I am looking
at one last issue in addition to this mutex problem. I have dmesg logs
to send and I will send them when I send the patches, so you have code
to look at as well as the dmesg.

thanks,
-- Shuah

-- 
Shuah Khan
Sr. Linux Kernel Developer
Open Source Innovation Group
Samsung Research America (Silicon Valley)
shuahkh@osg.samsung.com | (970) 217-8978