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

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 1/23/23 11:17, Richard Fitzgerald wrote:
> 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.

Like I said before, the Intel code will work with either b) or c).

Using both to exit clock stop is not a recommended/tested solution, and
it's not something I have a burning desire to look into. If you register
an external IRQ, then pretty please describe your device as not
'wake_capable'.

>> 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.