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

[Sowjanya Komatineni <skomatineni@nvidia.com>]

On 4/8/20 12:38 PM, Sowjanya Komatineni wrote:
>
> 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.
>
>
>
Tested with 3 buffers and by checking outstanding buffers in process by 
VI hw and holding to start capture till one outstanding buffer in 
process by HW.
Also tested with 2 buffers without checking for outstanding buffers.

In both cases, I see same %CPU for the kthreads and is < 1%