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

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Message-ID: <97b35910-4c93-123a-43a0-eb14476ed0f3@nvidia.com>
Date:   Mon, 6 Apr 2020 16:48:41 -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 4:18 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 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.
we hold on to issuing capture if more than 2 buffers are queued and it 
continues only after fifo has min 1 slot empty