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

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Message-ID: <05a00da2-2ff8-b234-3959-b451849b8cdb@opensource.cirrus.com>
Date:   Mon, 23 Jan 2023 16:08:02 +0000
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        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 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.