linux-kernel - Re: [PATCH v2 6/9] ALSA: virtio: PCM substream operators

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Date:   Tue, 26 Jan 2021 08:25:58 +0100 (CET)
From:   Guennadi Liakhovetski <guennadi.liakhovetski@...ux.intel.com>
To:     Guennadi Liakhovetski <guennadi.liakhovetski@...ux.intel.com>
cc:     Anton Yakovlev <anton.yakovlev@...nsynergy.com>,
        virtualization@...ts.linux-foundation.org,
        alsa-devel@...a-project.org, virtio-dev@...ts.oasis-open.org,
        linux-kernel@...r.kernel.org, Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2 6/9] ALSA: virtio: PCM substream operators

One more thing I missed yesterday:

On Mon, 25 Jan 2021, Guennadi Liakhovetski wrote:

>
> On Sun, 24 Jan 2021, Anton Yakovlev wrote:
>
>> Introduce the operators required for the operation of substreams.
>> 
>> Signed-off-by: Anton Yakovlev <anton.yakovlev@...nsynergy.com>
>> ---
>> sound/virtio/Makefile         |   3 +-
>> sound/virtio/virtio_pcm.c     |   5 +-
>> sound/virtio/virtio_pcm.h     |   2 +
>> sound/virtio/virtio_pcm_ops.c | 513 ++++++++++++++++++++++++++++++++++
>> 4 files changed, 521 insertions(+), 2 deletions(-)
>> create mode 100644 sound/virtio/virtio_pcm_ops.c
>
> [snip]
>
>> diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
>> new file mode 100644
>> index 000000000000..19882777fcd6
>> --- /dev/null
>> +++ b/sound/virtio/virtio_pcm_ops.c
>> @@ -0,0 +1,513 @@
>
> [snip]
>
>> +/**
>> + * virtsnd_pcm_release() - Release the PCM substream on the device side.
>> + * @substream: VirtIO substream.
>> + *
>> + * Context: Any context that permits to sleep.
>> + * Return: 0 on success, -errno on failure.
>> + */
>> +static inline bool virtsnd_pcm_released(struct virtio_pcm_substream 
>> *substream)
>> +{
>> +	/*
>> +	 * The spec states that upon receipt of the RELEASE command "the 
>> device
>> +	 * MUST complete all pending I/O messages for the specified stream 
>> ID".
>> +	 * Thus, we consider the absence of I/O messages in the queue as an
>> +	 * indication that the substream has been released.
>> +	 */
>> +	return atomic_read(&substream->msg_count) == 0;
>
> Also here having it atomic doesn't really seem to help. This just means, that 
> at some point of time it was == 0.
>
>> +}
>> +
>> +static int virtsnd_pcm_release(struct virtio_pcm_substream *substream)
>
> kernel-doc missing
>
>> +{
>> +	struct virtio_snd *snd = substream->snd;
>> +	struct virtio_snd_msg *msg;
>> +	unsigned int js = msecs_to_jiffies(msg_timeout_ms);
>> +	int rc;
>> +
>> +	msg = virtsnd_pcm_ctl_msg_alloc(substream, VIRTIO_SND_R_PCM_RELEASE,
>> +					GFP_KERNEL);
>> +	if (IS_ERR(msg))
>> +		return PTR_ERR(msg);
>> +
>> +	rc = virtsnd_ctl_msg_send_sync(snd, msg);
>> +	if (rc)
>> +		return rc;
>> +
>> +	return wait_event_interruptible_timeout(substream->msg_empty,
>> +						virtsnd_pcm_released(substream),
>> +						js);

wait_event_interruptible_timeout() will return a positive number in 
success cases, 0 means a timeout and condition still false. Whereas when 
you call this function you interpret 0 as success and you expect any != 0 
to be a negative error. Wondering how this worked during your tests?

Thanks
Guennadi