Re: [PATCH 5/9] soc: apple: Add RTKit IPC library

[Arnd Bergmann <arnd@arndb.de>]

On Sat, Apr 2, 2022 at 2:56 PM Sven Peter <sven@svenpeter.dev> wrote:
> On Tue, Mar 22, 2022, at 14:13, Arnd Bergmann wrote:
> >> +static int apple_rtkit_worker(void *data)
> >> +{
> >> +       struct apple_rtkit *rtk = data;
> >> +       struct apple_rtkit_work work;
> >> +
> >> +       while (!kthread_should_stop()) {
> >> +               wait_event_interruptible(rtk->wq,
> >> +                                        kfifo_len(&rtk->work_fifo) > 0 ||
> >> +                                                kthread_should_stop());
> >> +
> >> +               if (kthread_should_stop())
> >> +                       break;
> >> +
> >> +               while (kfifo_out_spinlocked(&rtk->work_fifo, &work, 1,
> >> +                                           &rtk->work_lock) == 1) {
> >> +                       switch (work.type) {
> >> +                       case APPLE_RTKIT_WORK_MSG:
> >> +                               apple_rtkit_rx(rtk, &work.msg);
> >> +                               break;
> >> +                       case APPLE_RTKIT_WORK_REINIT:
> >> +                               apple_rtkit_do_reinit(rtk);
> >> +                               break;
> >> +                       }
> >> +               }
> >
> > It looks like you add quite a bit of complexity by using a custom
> > worker thread implementation. Can you explain what this is
> > needed for? Isn't this roughly the same thing that one would
> > get more easily with create_singlethread_workqueue()?
>
> I originally had just a workqueue here but I can only put
> one instance of e.g. APPLE_RTKIT_WORK_MSG onto these.
> There could however be a new incoming message while the previous
> one is still being handled and I couldn't figure out a way
> to handle that with workqueues without introducing a race.

Are you trying to avoid dynamic allocation of the messages then
and have no other place that you can embed it in?

If you kmalloc() a messages that embeds a work_struct, you can
enqueue as many of those as you want, but the allocation adds
complexity through the need for error handling etc.

I wonder if you can change the mailbox driver to use a threaded
irq handler, which I think should ensure that the callback here
is run in process context, avoiding the need to defer execution
within the rtkit driver.

         Arnd