Re: [PATCH v6 3/9] ALSA: virtio: handling control messages

From: Takashi Iwai <tiwai@suse.de>
To: Anton Yakovlev <anton.yakovlev@opensynergy.com>
Cc: <virtualization@lists.linux-foundation.org>,
	<alsa-devel@alsa-project.org>, <virtio-dev@lists.oasis-open.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Jaroslav Kysela <perex@perex.cz>, Takashi Iwai <tiwai@suse.com>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 3/9] ALSA: virtio: handling control messages
Date: Sun, 28 Feb 2021 12:04:41 +0100	[thread overview]
Message-ID: <s5hv9acjvw6.wl-tiwai@suse.de> (raw)
In-Reply-To: <20210227085956.1700687-4-anton.yakovlev@opensynergy.com>

On Sat, 27 Feb 2021 09:59:50 +0100,
Anton Yakovlev wrote:
> 
> --- a/sound/virtio/virtio_card.c
> +++ b/sound/virtio/virtio_card.c
> @@ -11,6 +11,10 @@
>  
>  #include "virtio_card.h"
>  
> +int msg_timeout_ms = MSEC_PER_SEC;
> +module_param(msg_timeout_ms, int, 0644);
> +MODULE_PARM_DESC(msg_timeout_ms, "Message completion timeout in milliseconds");

If it's a global variable, better to set a prefix to make it unique,
and use module_param_named().

And, it should be unsigned int, no? (unless a negative value has any meaning)
Otherwise...

> +	if (!msg_timeout_ms) {
> +		dev_err(&vdev->dev, "msg_timeout_ms value cannot be zero\n");
> +		return -EINVAL;

Here a negative value would pass.

(snip)
> +int virtsnd_ctl_msg_send(struct virtio_snd *snd, struct virtio_snd_msg *msg,
> +			 struct scatterlist *out_sgs,
> +			 struct scatterlist *in_sgs, bool nowait)
> +{
> +	struct virtio_device *vdev = snd->vdev;
> +	struct virtio_snd_queue *queue = virtsnd_control_queue(snd);
> +	unsigned int js = msecs_to_jiffies(msg_timeout_ms);
> +	struct virtio_snd_hdr *request = virtsnd_ctl_msg_request(msg);
> +	struct virtio_snd_hdr *response = virtsnd_ctl_msg_response(msg);
> +	unsigned int nouts = 0;
> +	unsigned int nins = 0;
> +	struct scatterlist *psgs[4];
> +	bool notify = false;
> +	unsigned long flags;
> +	int rc;
> +
> +	virtsnd_ctl_msg_ref(msg);
> +
> +	/* Set the default status in case the message was canceled. */
> +	response->code = cpu_to_le32(VIRTIO_SND_S_IO_ERR);
> +
> +	psgs[nouts++] = &msg->sg_request;
> +	if (out_sgs)
> +		psgs[nouts++] = out_sgs;
> +
> +	psgs[nouts + nins++] = &msg->sg_response;
> +	if (in_sgs)
> +		psgs[nouts + nins++] = in_sgs;
> +
> +	spin_lock_irqsave(&queue->lock, flags);
> +	rc = virtqueue_add_sgs(queue->vqueue, psgs, nouts, nins, msg,
> +			       GFP_ATOMIC);

It's a bit pity that we have to use GFP_ATOMIC always here...
As long as it's in spinlock, it's the only way.

However, this reminds me of another question: may the virtio event be
handled in an atomic context, e.g. the period elapsed or xrun events?
If so, the current implementation with non-atomic PCM mode is wrong.
Since the non-atomic PCM uses mutex instead of spinlock, it'll lead to
a sleep-in-atomic in snd_pcm_period_elapsed() handling.

I suggested the non-atomic PCM *iff* the all contexts are sleepable;
then the sync can be done in each callback, which makes the code much
simpler usually.  But you've already implemented the sync via
sync_stop call, hence the non-atomic PCM has little benefit by its
own.

thanks,

Takashi