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

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Date:   Mon, 6 Apr 2020 15:07:53 -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 3:00 PM, Sowjanya Komatineni wrote:
>
> 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.

w.r.t capture list update, wakeup happens when wake_up on waitqueue is 
called.

wakeup also happens on kthread stop signal event.

>
>
>>>
>>> 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.
>
>
>