The commit d215f63d49da ("ALSA: usb-audio: Check available frames for
the next packet size") introduced the available frame size check, but
the conversion forgot to initialize the temporary variable properly,
and it resulted in a bogus calculation.  This patch fixes it.

Fixes: d215f63d49da ("ALSA: usb-audio: Check available frames for the next packet size")
Reported-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20211001104417.14291-1-colin.king@canonical.com
Link: https://lore.kernel.org/r/20211001105425.16191-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

While draining a stream, ALSA PCM core stops the stream by issuing
snd_pcm_stop() after all data has been sent out.  And, at PCM trigger
stop, currently USB-audio driver kills the in-flight URBs explicitly,
then at sync-stop ops, sync with the finish of all remaining URBs.
This might result in a drop of the drained samples as most of
USB-audio devices / hosts allow relatively long in-flight samples (as
a sort of FIFO).

For avoiding the trimming, this patch changes the stream-stop behavior
during PCM draining state.  Under that condition, the pending URBs
won't be killed.  The leftover in-flight URBs are caught by the
sync-stop operation that shall be performed after the trigger-stop
operation.

Link: https://lore.kernel.org/r/20210929080844.11583-10-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

This is another attempt to improve further the handling of playback
stream in the low latency mode.  The latest workaround in commit
4267c5a8f313 ("ALSA: usb-audio: Work around for XRUN with low latency
playback") revealed that submitting URBs forcibly in advance may
trigger XRUN easily.  In the classical mode, this problem was avoided
by practically delaying the submission of the actual data with the
pre-submissions of silent data before triggering the stream start.
But that is exactly what we want to avoid.

Now, in this patch, instead of the previous workaround, we take a
similar approach as used in the implicit feedback mode.  The URBs are
queued at the PCM trigger start like before, but we check whether the
buffer has been already filled enough before each submission, and
stop queuing if the data overcomes the threshold.  The remaining URBs
are kept in the ready list, and they will be retrieved in the URB
complete callback of other (already queued) URBs.  In the complete
callback, we try to fill the data and submit as much as possible
again.  When there is no more available in-flight URBs that may handle
the pending data, we'll check in PCM ack callback and submit and
process URBs there in addition.  In this way, the amount of in-flight
URBs may vary dynamically and flexibly depending on the available data
without hitting XRUN.

The following things are changed to achieve the behavior above:

* The endpoint prepare callback is changed to return an error code;
  when there is no enough data available, it may return -EAGAIN.
  Currently only prepare_playback_urb() returns the error.

  The evaluation of the available data is a bit messy here; we can't
  check with snd_pcm_avail() at the point of prepare callback (as
  runtime->status->hwptr hasn't been updated yet), hence we manually
  estimate the appl_ptr and compare with the internal hwptr_done to
  calculate the available frames.

* snd_usb_endpoint_start() doesn't submit full URBs if the prepare
  callback returns -EAGAIN, and puts the remaining URBs to the ready
  list for the later submission.

* snd_complete_urb() treats the URBs in the low-latency mode similarly
  like the implicit feedback mode, and submissions are done in
  (now exported) snd_usb_queue_pending_output_urbs().

* snd_usb_queue_pending_output_urbs() again checks the error value
  from the prepare callback.  If it's -EAGAIN for the normal stream
  (i.e. not implicit feedback mode), we push it back to the ready list
  again.

* PCM ack callback is introduced for the playback stream, and it calls
  snd_usb_queue_pending_output_urbs() if there is no in-flight URB
  while the stream is running.  This corresponds to the case where the
  system needs the appl_ptr update for re-submitting a new URB.

* snd_usb_queue_pending_output_urbs() and the prepare EP callback
  receive in_stream_lock argument, which is a bool flag indicating the
  call path from PCM ack.  It's needed for avoiding the deadlock of
  snd_pcm_period_elapsed() calls.

* Set the new SNDRV_PCM_INFO_EXPLICIT_SYNC flag when the new
  low-latency mode is deployed.  This assures catching each applptr
  update even in the mmap mode.

Fixes: 4267c5a8f313 ("ALSA: usb-audio: Work around for XRUN with low latency playback")
Link: https://lore.kernel.org/r/20210929080844.11583-9-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

In theory, stop_urbs() may be called concurrently.
Although we have the state check beforehand, it's safer to apply
ep->lock during the critical list head manipulations.

Link: https://lore.kernel.org/r/20210929080844.11583-8-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

This is yet more preparation for the upcoming changes.

Extend snd_usb_endpoint_next_packet_size() to check the available
frames and return -EAGAIN if the next packet size is equal or exceeds
the given size.  This will be needed for avoiding XRUN during the low
latency operation.

As of this patch, avail=0 is passed, i.e. the check is skipped and no
behavior change.

Link: https://lore.kernel.org/r/20210929080844.11583-7-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

This is a preparation patch for the upcoming low-latency improvement
changes.

Rename early_playback_start flag with lowlatency_playback as it's more
intuitive.  The new flag is basically a reverse meaning.

Along with the rename, factor out the code to set the flag to a
function.  This makes the complex condition checks simpler.

Also, the same flag is introduced to snd_usb_endpoint, too, that is
carried from the snd_usb_substream flag.  Currently the endpoint flag
isn't still referred, but will be used in later patches.

Link: https://lore.kernel.org/r/20210929080844.11583-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

USB-audio driver tries to sync with the clear of all pending URBs in
wait_clear_urbs(), and it waits for all bits in active_mask getting
cleared.  This works fine for the normal operations, but when a stream
is managed in the implicit feedback mode, there is still a very thin
race window: namely, in snd_complete_usb(), the active_mask bit for
the current URB is once cleared before re-submitted in
queue_pending_output_urbs().  If wait_clear_urbs() is called during
that period, it may pass the test and go forward even though there may
be a still pending URB.

For covering it, this patch adds a new counter to each endpoint to
keep the number of in-flight URBs, and changes wait_clear_urbs()
checking this number instead.  The counter is decremented at the end
of URB complete, hence the reference is kept as long as the URB
complete is in process.

Link: https://lore.kernel.org/r/20210929080844.11583-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

When a single clock source is shared among several endpoints, we have
to keep the same rate on all active endpoints as long as the clock is
being used.  For dealing with such a case, this patch adds one more
check in the hw params constraint for the rate to take the shared
clocks into account.  The current rate is evaluated from the endpoint
list that applies the same clock source.

BugLink: https://bugzilla.suse.com/show_bug.cgi?id=1190418
Link: https://lore.kernel.org/r/20210929080844.11583-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

The recent change for low latency playback works in most of test cases
but it turned out still to hit errors on some use cases, most notably
with JACK with small buffer sizes.  This is because USB-audio driver
fills up and submits full URBs at the beginning, while the URBs would
return immediately and try to fill more -- that can easily trigger
XRUN.  It was more or less expected, but in the small buffer size, the
problem became pretty obvious.

Fixing this behavior properly would require the change of the
fundamental driver design, so it's no trivial task, unfortunately.
Instead, here we work around the problem just by switching back to the
old method when the given configuration is too fragile with the low
latency stream handling.  As a threshold, we calculate the total
buffer bytes in all plus one URBs, and check whether it's beyond the
PCM buffer bytes.  The one extra URB is needed because XRUN happens at
the next submission after the first round.

Fixes: 307cc9baac5c ("ALSA: usb-audio: Reduce latency at playback start, take#2")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210827203311.5987-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>