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

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Message-ID: <9e317f65-8a02-3b15-cfec-8e0d8374130e@gmail.com>
Date:   Tue, 7 Apr 2020 02:18:04 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.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

07.04.2020 01:07, Sowjanya Komatineni пишет:
> 
> 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.

IIUC, the waiting won't happen if more than 2 captures are queued and
thread will be spinning until captures are processed.

I think you need a semaphore with resource count = 2.