On Fri, Aug 12, 2016 at 2:03 PM, Takashi Iwai <tiwai@xxxxxxx> 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
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