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

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Date:   Mon, 6 Apr 2020 13:20:46 -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 1:02 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?
>
> 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.

>
>> +             /*
>> +              * Frame start and MW_ACK_DONE syncpoint condition FIFOs are
>> +              * of max depth 2. So make sure max 2 capture requests are
>> +              * in process by the hardware at a time.
>> +              */
>> +             while (!(kthread_should_stop() || list_empty(&chan->capture))) {
>> +                     caps_inflight = chan->capture_reqs - chan->sequence;
>> +                     /*
>> +                      * Source is not streaming if error is non-zero.
>> +                      * So, do not dequeue buffers on capture error or when
>> +                      * syncpoint requests in FIFO are full.
>> +                      */
>> +                     if (err || caps_inflight >= SYNCPT_FIFO_DEPTH)
>> +                             break;
>> +
>> +                     /* dequeue the buffer and start capture */
>> +                     spin_lock(&chan->start_lock);
>> +                     if (list_empty(&chan->capture))
>> +                             break;
>> +                     buf = list_entry(chan->capture.next,
>> +                                      struct tegra_channel_buffer, queue);
> list_first_entry()
>
>> +                     list_del_init(&buf->queue);
>> +                     spin_unlock(&chan->start_lock);
>> +
>> +                     err = tegra_channel_capture_frame(chan, buf);
>> +                     if (err)
>> +                             vb2_queue_error(&chan->queue);
>> +             }
>> +
>> +             if (kthread_should_stop())
>> +                     break;
>> +     }
>> +
>> +     return 0;
>> +}