Message-ID: <8a05bb50-9743-d3cc-cff7-8b93aa1f68df@opensource.cirrus.com>
Date:   Mon, 23 Jan 2023 17:17:52 +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 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.

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