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

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Message-ID: <fea4f0a1-4a20-34d4-9eda-e4a599eeeffc@nvidia.com>
Date:   Mon, 6 Apr 2020 15:00:43 -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 4/6/20 2:39 PM, Sowjanya Komatineni wrote:
>
> On 4/6/20 2:15 PM, Sowjanya Komatineni wrote:
>>
>> On 4/6/20 2:11 PM, Dmitry Osipenko wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 07.04.2020 00:02, Sowjanya Komatineni пишет:
>>>>>>>>> Am I understanding correctly that this thread will take 100% CPU,
>>>>>>>>> spinning here, if more than 2 frame-captures queued?
>>>>>>>> on more than 2 frames captures, it breaks thread and on next 
>>>>>>>> wakeup it
>>>>>>>> continues
>>>>>>> The wait_event() won't wait if condition is true.
>>>>>> condition is checked when waitqueue is woken up
>>>>> https://elixir.bootlin.com/linux/v5.6.2/source/include/linux/wait.h#L462 
>>>>>
>>>> process is put to sleep until the condition evaluates to true or 
>>>> signal
>>>> is received.
>>>>
>>>> condition is checked each time the waitqueue head is woken up.
>>> This is a wrong assumption in accordance to the code.

process is in sleep until the condition is evaluated and when condition 
is true wakeup still happens only when wake_up on waitqueue is called

This is the reason for using this to prevent blocking while waiting for 
the buffers.

>>
>> when every buffer is available as long as we are in streaming, we 
>> should process it.
>>
>> So if wake up happens when list has buffer, it will be processed but 
>> at a time we limit processing 2 simultaneous buffer capture starts only.
>>
> Fixing typo.
>
> I meant when ever buffer is available as long as we are in streaming, 
> we should process it.
>
> So capture thread processes as long as buffers are available from user 
> space limiting 2 simultaneous trigger of captures and thread will be 
> in sleep when capture buffers list is empty or no stop thread is 
> signaled.