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@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20230317195128.3911155-1-john@xxxxxxxxxxxx
> Cc: Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
> 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@xxxxxxxxxxxxx>
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);
> return 0;
> }
>
> --
> 2.35.3
Thanks
Takashi Sakamoto
