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

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Message-ID: <a638bb8e-bb50-7aa5-05a0-8de1c6207ba7@nvidia.com>
Date:   Wed, 8 Apr 2020 11:58:52 -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 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.