Re: [RFC PATCH v11 6/9] media: tegra: Add Tegra210 Video input driver

From: Sowjanya Komatineni
Date: Thu Apr 30 2020 - 17:38:34 EST


On 4/30/20 2:26 PM, Sowjanya Komatineni wrote:

On 4/30/20 2:17 PM, Dmitry Osipenko wrote:
30.04.2020 23:02, Sowjanya Komatineni ÐÐÑÐÑ:
On 4/30/20 12:53 PM, Sowjanya Komatineni wrote:
On 4/30/20 12:46 PM, Sowjanya Komatineni wrote:
On 4/30/20 12:33 PM, Dmitry Osipenko wrote:
30.04.2020 22:09, Sowjanya Komatineni ÐÐÑÐÑ:
On 4/30/20 11:18 AM, Sowjanya Komatineni wrote:
On 4/30/20 10:06 AM, Sowjanya Komatineni wrote:
On 4/30/20 9:29 AM, Sowjanya Komatineni wrote:
On 4/30/20 9:04 AM, Sowjanya Komatineni wrote:
On 4/30/20 7:13 AM, Dmitry Osipenko wrote:
30.04.2020 17:02, Dmitry Osipenko ÐÐÑÐÑ:
30.04.2020 16:56, Dmitry Osipenko ÐÐÑÐÑ:
30.04.2020 01:00, Sowjanya Komatineni ÐÐÑÐÑ:
+static int chan_capture_kthread_finish(void *data)
+{
+ÂÂÂ struct tegra_vi_channel *chan = data;
+ÂÂÂ struct tegra_channel_buffer *buf;
+
+ÂÂÂ set_freezable();
+
+ÂÂÂ while (1) {
+ÂÂÂÂÂÂÂ try_to_freeze();
I guess it won't be great to freeze in the middle of a capture
process, so:
ÂÂÂÂÂÂÂÂÂ if (list_empty(&chan->done))
ÂÂÂÂÂÂÂÂÂÂÂÂÂ try_to_freeze();
And here should be some locking protection in order not race
with
the
chan_capture_kthread_start because kthread_finish could freeze
before
kthread_start.
Or maybe both start / finish threads should simply be allowed to
freeze
only when both capture and done lists are empty.

if (list_empty(&chan->capture) &&
ÂÂÂÂÂÂ list_empty(&chan->done))
ÂÂÂÂÂÂtry_to_freeze();
good to freeze when not in middle of the frame capture but why
should we not allow freeze in between captures?

Other drivers do allow freeze in between frame captures.

I guess we can freeze before dequeue for capture and in finish
thread we can freeze after capture done. This also don't need to
check for list_empty with freeze to allow between frame captures.

Also if we add check for both lists empty, freeze is not allowed as
long as streaming is going on and in case of continuous streaming
freeze will never happen.
To allow freeze b/w frames (but not in middle of a frame),

for capture_start thread, probably we can do unconditional
try_to_freeze()
Is it possible to use wait_event_freezable()?

https://www.kernel.org/doc/Documentation/power/freezing-of-tasks.txt

Will the wait_event_interruptible() be woken up when system freezes?
Based on wait_event_freezable implementation, looks like it similar
to wait_event_interruptible + try_to_free() as it does
freezable_schedule unlike schedule with wait_event_interruptible.

So using this for capture_start may be ok to allow freeze before
start of frame. But can't use for capture_finish as this is same as
wait_event_interruptible followed by unconditional try_to_freeze.

for capture_finish thread, at end of capture done we can do
try_to_freeze() only when done list is empty
This doesn't prevent situation where the done-list is empty and the
"finish" thread freezes, in the same time the "start" thread issues new
capture and then freezes too.

1. "start" thread issues capture

2. "finish" thread wakes and waits for the capture to complete

3. "start" thread begins another capture, waits for FRAME_START

4. system freezing activates

5. "finish" thread completes the capture and freezes because done-list
is empty

6. "start" thread gets FRAME_START, issues another capture and freezes
This will not happen as we allow double buffering done list will not
be empty till stream stop happens

There will always be 1 outstanding frame in done list
Correction, there will always be 1 outstanding buffer except beginning
during beginning of stream.

Except during beginning frames, done list will not be empty for all
subsequent streaming process
Also to be clear, hardware sees next frame start event prior to previous
frame mw_ack event as mw_ack event happens after frame end. So once
initial buffer got queued to done list to finish processes, while
waiting for mw_ack next frame start happens and pushes next buffer to
done list.
What about this variant:

1. "start" thread wakes up to start capture

2. system freezing activates

3. "finish" thread wakes up and freezes

finish thread will wake up only when done list is not empty/kthread_stop/wake even from capture start thread.

Also when I said will allow try_to_free when done list is empty I meant to have this at end of capture_done() in finish thread


4. "start" thread issues capture and freezes

And again, I assume that system's freezing should wake
wait_event_interruptible(), otherwise it won't be possible to freeze
idling threads at all and freezing should fail (IIUC).

Based on kernel doc on freezing, looks like when we mark thread as freezable, freeze state happens when we explicitly call try_to_freeze.

I don't think its other way where freeze causes wait_event_interruptible to wake up.

And in this case synchronization between start/finish threads should be
needed in regards to freezing.

Was thinking to have counter to track outstanding frame w.r.t single shot issue b/w start and finish and allow to freeze only when no outstanding frames in process.

This will make sure freeze will not happen when any buffers are in progress

Note that this could be a wrong assumption, I'm not closely familiar
with how freezer works.

kthread_start can unconditionally allow try_to_freeze before start of frame capture

We can compute captures inflight w.r.t single shot issued during capture start and finished frames by kthread_finish and allow kthread_finish to freeze only when captures inflight is 0.

This allows freeze to happen b/w frames but not in middle of frame