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

[Richard Fitzgerald]

On 23/01/2023 16:38, Pierre-Louis Bossart wrote:
> On 1/23/23 10:08, Richard Fitzgerald wrote:
> > 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.
>
> The Intel hardware has a power optimization for the clock-stop, which
> leads to different paths to wake the system. The SoundWire IP can deal
> with the data line staying high, but in the optimized mode the wakes are
> signaled as DSP interrupts at a higher level. That's why we added this
> intel_link_process_wakeen_event() function called from
> hda_dsp_interrupt_thread().
>
> So yes on paper everything would work nicely, but that's asking for
> trouble with races left and right. In other words, unless you have a

Wake up from a hard INT is simply a runtime_resume of the codec driver.
That is no different from ASoC runtime resuming the driver to perform
some audio activity, or to access a volatile register. An event caused
a runtime-resume - the driver and the host controller must resume.

The Intel code _must_ be able to safely wakeup from clock-stop if
something runtime-resumes the codec driver. ASoC relies on that, and
pm_runtime would be broken if that doesn't work.

> very good reason for using two wake-up mechanisms, pick a single one.
>
> (a) and (c) are very similar in that all the exit is handled by
> pm_runtime so I am not worried too much. I do worry about paths that
> were never tested and never planned for.