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

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Date:   Tue, 7 Apr 2020 14:08:36 -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 4:48 PM, Sowjanya Komatineni wrote:
>
> 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


Just want to close on this part of feedback. Hope above explanation is 
clear regarding triggering/issuing at max 2 frame capture to VI HW and 
also regarding capture threads where they use wait_event_interruptible 
to prevent blocking waiting for buffers to be available for captures.

So no changes related to this part are needed in v7.