linux-kernel - Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver

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Date:   Wed, 8 Apr 2020 12:38:21 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>, <thierry.reding@...il.com>,
        <jonathanh@...dia.com>, <frankc@...dia.com>, <hverkuil@...all.nl>,
        <sakari.ailus@....fi>, <helen.koike@...labora.com>
CC:     <sboyd@...nel.org>, <linux-media@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-clk@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver


On 4/8/20 11:58 AM, Sowjanya Komatineni wrote:
>
> On 4/8/20 10:45 AM, Sowjanya Komatineni wrote:
>>
>> On 4/8/20 7:21 AM, Dmitry Osipenko wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 08.04.2020 03:00, Sowjanya Komatineni пишет:
>>> ...
>>>>>>>> I suppose that taking a shot takes at least few milliseconds, 
>>>>>>>> which
>>>>>>>> should be unacceptable to waste.
>>>>>>> As long as buffers are in queue we have to keep processing each
>>>>>>> buffer and between buffers obviously we have to wait for previous
>>>>>>> frames to finish and this why we have separate thread for frame
>>>>>>> finish where we can have next buffer capture ready and issue while
>>>>>>> previous frame memory write happens
>>>>> Also we specified numbers buffers as 3 to vb2 queue. So this is rare
>>>>> case but to prevent issuing more than 2 at a time as VI HW is only
>>>>> double buffered and syncpt fifo max depth is 2 added this to be 
>>>>> safer.
>>>> To be more clear, when more buffers are enqueued from userspace always
>>>> capture list will be full and thread will be busy in capture till 
>>>> either
>>>> error or stop stream request happens.
>>>>
>>> If kthreads take more than 1% of CPU time during capture (video) with
>>> more than 2 buffers in queue, then it's not good and I think you should
>>> do something about it. If kthreads stay at ~0%, then it should be okay
>>> as-is.
>>
>> VI outstanding requests max can only be 2  as syncpt fifo depth is 2  
>> and waiting to issue next capture when already 2 captures are 
>> inflight happens only during beginning of streaming where buffers 
>> allocated go thru capture for first time after queuing.
>>
>> same buffers are returned to userspace after capture and same 
>> allocated buffers will be queued back for subsequent captures.
>>
>> So this case of holding to issue single shot when already single shot 
>> is issue for 2 frames simultaneous happens only during beginning of 
>> start stream and also we set num_buffers to allocate for queue as 3 
>> although 2 is good enough where we will not hit this case even during 
>> streaming start with 2 buffers
>>
> As 2 buffers are good enough to be clear will update in v7 to use 2 
> buffers so we don't need to check for more than 2 outstanding buffers.

correction: With 3 buffers, as soon as buffer is available capture 
starts. So right most times I see it waiting for few ms before 3rd 
capture to get through.

As only 2 frames single shot can be issued in sequence (inflight 
requests), instead of waiting for 1 of the request to finish, we can use 
2 buffers and avoid waiting as 2 buffers are good enough. Will change 
this in v7.