On Tue, Mar 21, 2023 at 01:02:58PM +0900, Takashi Sakamoto wrote:
Hi,
On Mon, Mar 20, 2023 at 03:28:38PM +0100, Takashi Iwai wrote:
The recent support of low latency playback in USB-audio driver made the snd_usb_queue_pending_output_urbs() function to be called via PCM ack ops. In the new code path, the function is performed alread in
's/alread/already/' or slang.
the PCM stream lock. The problem is that, when an XRUN is detected, the function calls snd_pcm_xrun() to notify, but snd_pcm_xrun() is
's/the function/the function/'
supposed to be called only outside the stream lock. As a result, it leads to a deadlock of PCM stream locking.
For avoiding such a recursive locking, this patch adds an additional check to the code paths in PCM core that call the ack callback; now it checks the error code from the callback, and if it's -EPIPE, the XRUN is handled in the PCM core side gracefully. Along with it, the USB-audio driver code is changed to follow that, i.e. -EPIPE is returned instead of the explicit snd_pcm_xrun() call when the function is performed already in the stream lock.
Practically, the implementation of 'pcm_hw' in alsa-lib never evaluates the return value (see 'snd_pcm_hw_mmap_commit()' and the others). I guess that it is inconvenient for the low-latency mode of USB Audio device class driver for the case of failure.
My additional concern is PCM indirect layer, since typically the layer is used by drivers with SNDRV_PCM_INFO_SYNC_APPLPTR. But as long as I read, the change does not matter to them.
Fixes: d5f871f89e21 ("ALSA: usb-audio: Improved lowlatency playback support") Reported-and-tested-by: John Keeping john@metanate.com Link: https://lore.kernel.org/r/20230317195128.3911155-1-john@metanate.com Cc: Takashi Sakamoto o-takashi@sakamocchi.jp Signed-off-by: Takashi Iwai tiwai@suse.de
sound/core/pcm_lib.c | 2 ++ sound/usb/endpoint.c | 22 ++++++++++++++-------- sound/usb/endpoint.h | 4 ++-- sound/usb/pcm.c | 2 +- 4 files changed, 19 insertions(+), 11 deletions(-)
Anyway, John Keeping reports the change works well to solve the issue. I have no objection to it.
Reviewed-by; Takashi Sakamoto o-takashi@sakamocchi.jp
I forgot to mention that it is preferable to update 'ack callback' clause of API Documentation (i.e. writing-an-alsa-driver.rst) as well.
It is also convenient to IEC 61883-1/6 packet streaming engine for audio and music unit in IEEE 1394 bus. I will post patches enough later for it.
diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c index 8b6aeb8a78f7..02fd65993e7e 100644 --- a/sound/core/pcm_lib.c +++ b/sound/core/pcm_lib.c @@ -2155,6 +2155,8 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream, ret = substream->ops->ack(substream); if (ret < 0) { runtime->control->appl_ptr = old_appl_ptr;
if (ret == -EPIPE)__snd_pcm_xrun(substream); return ret;diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c index 419302e2057e..647fa054d8b1 100644 --- a/sound/usb/endpoint.c +++ b/sound/usb/endpoint.c @@ -455,8 +455,8 @@ static void push_back_to_ready_list(struct snd_usb_endpoint *ep,
- This function is used both for implicit feedback endpoints and in low-
- latency playback mode.
*/ -void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
bool in_stream_lock)+int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
bool in_stream_lock){ bool implicit_fb = snd_usb_endpoint_implicit_feedback_sink(ep);
@@ -480,7 +480,7 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep, spin_unlock_irqrestore(&ep->lock, flags);
if (ctx == NULL)
return;
break;/* copy over the length information */ if (implicit_fb) {
@@ -495,11 +495,14 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep, break; if (err < 0) { /* push back to ready list again for -EAGAIN */
if (err == -EAGAIN)
if (err == -EAGAIN) { push_back_to_ready_list(ep, ctx);
else
break;}if (!in_stream_lock) notify_xrun(ep);
return;
return -EPIPE;}
err = usb_submit_urb(ctx->urb, GFP_ATOMIC);
@@ -507,13 +510,16 @@ void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep, usb_audio_err(ep->chip, "Unable to submit urb #%d: %d at %s\n", ctx->index, err, __func__);
notify_xrun(ep);return;
if (!in_stream_lock)notify_xrun(ep);return -EPIPE;}
set_bit(ctx->index, &ep->active_mask); atomic_inc(&ep->submitted_urbs); }
return 0;
}
/* diff --git a/sound/usb/endpoint.h b/sound/usb/endpoint.h index 924f4351588c..c09f68ce08b1 100644 --- a/sound/usb/endpoint.h +++ b/sound/usb/endpoint.h @@ -52,7 +52,7 @@ int snd_usb_endpoint_implicit_feedback_sink(struct snd_usb_endpoint *ep); int snd_usb_endpoint_next_packet_size(struct snd_usb_endpoint *ep, struct snd_urb_ctx *ctx, int idx, unsigned int avail); -void snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
bool in_stream_lock);+int snd_usb_queue_pending_output_urbs(struct snd_usb_endpoint *ep,
bool in_stream_lock);#endif /* __USBAUDIO_ENDPOINT_H */ diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index d959da7a1afb..eec5232f9fb2 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -1639,7 +1639,7 @@ static int snd_usb_pcm_playback_ack(struct snd_pcm_substream *substream) * outputs here */ if (!ep->active_mask)
snd_usb_queue_pending_output_urbs(ep, true);
return snd_usb_queue_pending_output_urbs(ep, true);}
-- 2.35.3
Thanks
Takashi Sakamoto