Re: [PATCH 2/2] soundwire: bus: Allow SoundWire peripherals to register IRQ handlers

[Richard Fitzgerald]

On 23/01/2023 15:50, Pierre-Louis Bossart wrote:
> On 1/23/23 08:53, Charles Keepax wrote:
> > On Fri, Jan 20, 2023 at 10:20:50AM -0600, Pierre-Louis Bossart wrote:
> > > On 1/20/23 03:59, Charles Keepax wrote:
> > > > On Thu, Jan 19, 2023 at 11:12:04AM -0600, Pierre-Louis Bossart wrote:
> > > > > There should be an explanation and something checking that both are not
> > > > > used concurrently.
> > > >
> > > > I will try to expand the explanation a litte, but I dont see any
> > > > reason to block calling both handlers, no ill effects would come
> > > > for a driver having both and it is useful if any soundwire
> > > > specific steps are needed that arn't on other control buses.
> > >
> > > I think it's problematic if the peripheral tries to wake-up the manager
> > > from clock-stop with both an in-band wake (i.e. drive the data line
> > > high) and a separate GPIO-based interrupt. It's asking for trouble IMHO.
> > > We spent hours in the MIPI team to make sure there were no races between
> > > the manager-initiated restarts and peripheral-initiated restarts, adding
> > > a 3rd mechanism in the mix gives me a migraine already.
> >
> > Apologies but I am struggling see why this has any bearing on
> > the case of a device that does both an in-band and out-of-band
> > wake. The code we are adding in this patch will only be called in the
> > in-band case. handle_nested_irq doesn't do any hardware magic or
> > schedule any threads, it just calls a function that was provided
> > when the client called request_threaded_irq. The only guarantee
> > of atomicity you have on the interrupt_callback is sdw_dev_lock
> > and that is being held across both calls after the patch.
> >
> > Could you be a little more specific on what you mean by this
> > represents a 3rd mechanism, to me this isn't a new mechanism just
> > an extra callback? Say for example this patch added an
> > interrupt_callback_early to sdw_slave_ops that is called just
> > before interrupt_callback.
>
> Well, the main concern is exiting the clock-stop. That is handled by the
> manager and could be done
> a) as the result of the framework deciding that something needs to be
> done (typically as a result of user/applications starting a stream)
> b) by the device with an in-band wake in case of e.g. jack detection or
> acoustic events detected
> c) same as b) but with a separate out-of-band interrupt.
>
> I'd like to make sure b) and c) are mutually-exclusive options, and that
> the device will not throw BOTH an in-band wake and an external interrupt.

Why would it be a problem if the device did (b) and (c)?
(c) is completely invisible to the SoundWire core and not something
that it has to handle. The handler for an out-of-band interrupt must
call pm_runtime_get_sync() or pm_runtime_resume_and_get() and that
would wake its own driver and the host controller.