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

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Date:   Mon, 6 Apr 2020 13:43:26 -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 1:38 PM, Sowjanya Komatineni wrote:
>
> On 4/6/20 1:37 PM, Dmitry Osipenko wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> 06.04.2020 23:20, Sowjanya Komatineni пишет:
>>> On 4/6/20 1:02 PM, Dmitry Osipenko wrote:
>>>> External email: Use caution opening links or attachments
>>>>
>>>>
>>>> 04.04.2020 04:25, Sowjanya Komatineni пишет:
>>>> ...
>>>>> +static int chan_capture_kthread_start(void *data)
>>>>> +{
>>>>> +     struct tegra_vi_channel *chan = data;
>>>>> +     struct tegra_channel_buffer *buf;
>>>>> +     int err = 0;
>>>>> +     int caps_inflight;
>>>>> +
>>>>> +     set_freezable();
>>>>> +
>>>>> +     while (1) {
>>>>> +             try_to_freeze();
>>>>> +
>>>>> + wait_event_interruptible(chan->start_wait,
>>>>> + !list_empty(&chan->capture) ||
>>>>> + kthread_should_stop());
>>>> Is it really okay that list_empty() isn't protected with a lock?

wakeup on thread happens either when buffer is moved to capture list or 
on stop signaling event.

So in this specific case we may not need to check for lock on capture 
list as if wakeup happens from start wait queue, then buffer is already 
moved to capture list by then.

>>>>
>>>> Why wait_event is "interruptible"?
>>> To allow it to sleep until wakeup on thread it to avoid constant
>>> checking for condition even when no buffers are ready, basically to
>>> prevent blocking.
>> So the "interrupt" is for getting event about kthread_should_stop(),
>> correct?
> also to prevent blocking and to let is sleep and wakeup based on wait 
> queue to evaluate condition to proceed with the task
>>