Re: [PATCH v7 06/33] ASoC: Add SOC USB APIs for adding an USB backend

[Wesley Cheng]

Hi Mark,

On 9/27/2023 7:48 AM, Mark Brown wrote:
> On Thu, Sep 21, 2023 at 02:48:16PM -0700, Wesley Cheng wrote:
>
> > +static struct device_node *snd_soc_find_phandle(struct device *dev)
> > +{
> > +	struct device_node *node;
> > +
> > +	node = of_parse_phandle(dev->of_node, "usb-soc-be", 0);
>
> Very nitpicky but this function possibly wants a _usb_ in the name, not
> that it *super* matters with it being static.  Or it could just be
> inlined into the only user and not worry about the naming at all.

Thanks for the review!  Sure, let me reshuffle things around a bit and 
just get rid of this function entirely and inline it to the API below.

> > +/**
> > + * snd_soc_usb_get_priv_data() - Retrieve private data stored
> > + * @dev: device reference
> > + *
> > + * Fetch the private data stored in the USB SND SOC structure.
> > + *
> > + */
> > +void *snd_soc_usb_get_priv_data(struct device *dev)
> > +{
> > +	struct snd_soc_usb *ctx;
> > +
> > +	ctx = snd_soc_find_usb_ctx(dev);
> > +	if (!ctx) {
> > +		/* Check if backend device */
> > +		mutex_lock(&ctx_mutex);
> > +		list_for_each_entry(ctx, &usb_ctx_list, list) {
> > +			if (dev->of_node == ctx->dev->of_node) {
> > +				mutex_unlock(&ctx_mutex);
> > +				goto out;
> > +			}
> > +		}
> > +		mutex_unlock(&ctx_mutex);
> > +		ctx = NULL;
> > +	}
>
> This seems a lot more expensive than I'd expect for a get_priv_data
> operation, usually it's just a container_of() or other constant time
> pulling out of a pointer rather than a linked list walk - the sort of
> thing that people put at the start of functions and do all the time.
> If we need this I think it needs a name that's more clearly tied to the
> use case.
>
> I didn't actually find the user of this though?

So the end user of this would be the qc_audio_offload driver, within 
prepare_qmi_response().  This is to fetch some information about the 
DPCM backend during the stream enable request.

Previously, I limited the # of snd_soc_usb ports to be registered to 
one, but that would affect the scalability of this layer.  Hence, adding 
a list instead increased the complexity.  Will rename this accordingly.

Thanks
Wesley Cheng