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

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Date:   Mon, 23 Jan 2023 09:50:15 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Charles Keepax <ckeepax@...nsource.cirrus.com>
Cc:     alsa-devel@...a-project.org, patches@...nsource.cirrus.com,
        linux-kernel@...r.kernel.org, vkoul@...nel.org,
        sanyog.r.kale@...el.com, yung-chuan.liao@...ux.intel.com
Subject: Re: [PATCH 2/2] soundwire: bus: Allow SoundWire peripherals to
 register IRQ handlers



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.