Re: [PATCH 5/6] soudnwire: master: protect concurrecnt check for bus->md

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

On 20/04/2023 18:42, Pierre-Louis Bossart wrote:
> typos in commit title...
> 
> On 4/20/23 05:16, Krzysztof Kozlowski wrote:
>> The Soundwire master controllers might want to check for bus->md
> 
> Apologies for being pedantic but 'manager' and 'controller' are
> different concepts in SoundWire, see DisCo spec.
> It's not a 1:1 mapping, a controller can rely on M managers

I wrote master, not manager. For the Qualcomm case one controller is one
master, but in general I try to avoid the master/slave terminology.

> 
>> initialization to avoid race between early interrupt and finish of
>> sdw_bus_master_add()/sdw_master_device_add().  Such early interrupt can
>> happen if Soundwire devices are not powered off during their probe.
>>
>> Add a store release barrier, so the Soundwire controllers can safely
>> check it in concurrent (e.g. in interrupt) way.
> 
> Can you elaborate on the race condition? I am not following what breaks,
> and what entity generates the 'early interrupt'.

The condition is explained in next patch. If you think it's better, I
can squash it with next.

If the condition is still not clear, drop a note in next patch, so I
will elaborate there.

> 
> I am specifically concerned about adding this in common code without any
> matching smp_load_acquire() - which is only added in the following patch
> for the Qualcomm manager only, but not added for Intel/AMD managers. Is
> this not a problem?

Shouldn't be. The barrier just won't be effective for these drivers, but
that should not be a problem, because I also did not add to these
checking bus->md in a concurrent path.

Basically the barrier here is necessary because I want to check bus->md
in Qualcomm master interrupt handler.

Best regards,
Krzysztof