[alsa-devel] [PATCH 09/15] ALSA: line6: Add hwdep interface to access the POD control messages

[Takashi Iwai]

On Fri, 12 Aug 2016 13:10:37 +0200,
Andrej Kruták wrote:
> > > +static void line6_hwdep_push_message(struct usb_line6 *line6)
> > > +{
> > > +     unsigned long head = line6->buffer_circular.head;
> > > +     /* The spin_unlock() and next spin_lock() provide needed ordering. */
> > > +     unsigned long tail = ACCESS_ONCE(line6->buffer_circular.tail);
> > > +
> > > +     if (!line6->buffer_circular.active)
> > > +             return;
> > > +
> > > +     if (CIRC_SPACE(head, tail, LINE6_MESSAGE_MAXCOUNT) >= 1) {
> > > +             unsigned char *item = &line6->buffer_circular.data[
> > > +                     head * LINE6_MESSAGE_MAXLEN];
> > > +             memcpy(item, line6->buffer_message, line6->message_length);
> > > +             line6->buffer_circular.data_len[head] = line6->message_length;
> > > +
> > > +             smp_store_release(&line6->buffer_circular.head,
> > > +                               (head + 1) & (LINE6_MESSAGE_MAXCOUNT - 1));
> > > +             up(&line6->buffer_circular.sem);
> > > +     }
> >
> > Hmm...  this kind of a simple FIFO can be seen in anywhere in the
> > kernel code, and I'm sure that you can find an easier way to implement
> > it.  The whole code looks a bit scary as it being home-brewed.
> >
> 
> This code is based on Documentation/circular-buffers.txt, except for
> the semaphore magic.

The example there is basically a semi lock-free implementation.  For
your purpose it's an overkill.  This is no severely hot path, thus a
simpler version would make life easier.

> > Also, the blocking read/write control isn't usually done by a
> > semaphore.  Then you can handle the interrupt there.
> >
> >
> 
> I actually wonder why, semaphores seemed perfect for this... Do you
> have some hints?

Assume you want to interrupt the user-space app while it's being
blocked by the semaphore.  With your code, you can't.


Takashi