linux-kernel - Re: [PATCH v5 6/9] ALSA: virtio: PCM substream operators

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Date:   Thu, 25 Feb 2021 13:14:37 +0100
From:   Anton Yakovlev <anton.yakovlev@...nsynergy.com>
To:     Takashi Iwai <tiwai@...e.de>
CC:     <virtualization@...ts.linux-foundation.org>,
        <alsa-devel@...a-project.org>, <virtio-dev@...ts.oasis-open.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 6/9] ALSA: virtio: PCM substream operators

On 25.02.2021 11:55, Takashi Iwai wrote:
> On Mon, 22 Feb 2021 16:34:41 +0100,
> Anton Yakovlev wrote:
>> +static int virtsnd_pcm_open(struct snd_pcm_substream *substream)
>> +{
>> +     struct virtio_pcm *vpcm = snd_pcm_substream_chip(substream);
>> +     struct virtio_pcm_substream *vss = NULL;
>> +
>> +     if (vpcm) {
>> +             switch (substream->stream) {
>> +             case SNDRV_PCM_STREAM_PLAYBACK:
>> +             case SNDRV_PCM_STREAM_CAPTURE: {
> 
> The switch() here looks superfluous.  The substream->stream must be a
> good value in the callback.  If any, you can put WARN_ON() there, but
> I don't think it worth.

At least it doesn't do any harm. If something really went wrong, we can
check it right in the open callback, which is called the very first.


>> +static int virtsnd_pcm_hw_params(struct snd_pcm_substream *substream,
>> +                              struct snd_pcm_hw_params *hw_params)
>> +{
> ....
>> +     return virtsnd_pcm_msg_alloc(vss, periods, period_bytes);
> 
> We have the allocation, but...
> 
>> +static int virtsnd_pcm_hw_free(struct snd_pcm_substream *substream)
>> +{
>> +     return 0;
> 
> ... no release at hw_free()?
> I know that the free is present in the allocator, but it's only for
> re-allocation case, I suppose.

When the substream stops, sync_ptr waits until the device has completed
all pending messages. This wait can be interrupted either by a signal or
due to a timeout. In this case, the device can still access messages
even after calling hw_free(). It can also issue an interrupt, and the
interrupt handler will also try to access message structures. Therefore,
freeing of already allocated messages occurs either in hw_params() or in
dev->release(), since there it is 100% safe.


> thanks,
> 
> Takashi
> 

-- 
Anton Yakovlev
Senior Software Engineer

OpenSynergy GmbH
Rotherstr. 20, 10245 Berlin