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

[Andrej Kruták]

On Fri, Aug 12, 2016 at 2:03 PM, Takashi Iwai <tiwai at suse.de> wrote:
> 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.
>

Fair enough, I'll spinlock-ize it then.


>> > 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.
>
>

You can - down_interruptible() is there for this exact reason. As
said, semaphore is basically wait-queue + counter, so there's no real
reason not to do this IMO.

But in the end - if you have some nice example of FIFO buffer in some
simple driver, I have no problem using a more already-proven/standard
way :-)


-- 
Andrej