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

[Sowjanya Komatineni]

On 4/6/20 4:18 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
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>
> 07.04.2020 01:07, Sowjanya Komatineni ÐÐÑÐÑ:
> > 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 ÐÐÑÐÑ:
> > > > > > > > > > > > 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