Re: [PATCH] mfd: max77620: Avoid regmap mutex deadlock in power-off handler

[Dmitry Osipenko]

21.05.2026 12:19, Diogo Ivo пишет:
> 
> 
> On 5/20/26 18:44, Dmitry Osipenko wrote:
>> 20.05.2026 19:23, Mark Brown пишет:
>>> On Wed, May 20, 2026 at 05:19:00PM +0100, Lee Jones wrote:
>>>> On Wed, 20 May 2026, Diogo Ivo wrote:
>>>
>>>>> This patch was motivated by the Sashiko review I got in [1]. Its point
>>>>> here is that there is a possibility for a deadlock scenario in which
>>>>> a secondary CPU obtains the mutex for the regmap and then
>>>>> smp_send_stop()
>>>>> is called before this secondary CPU gets a chance to release the
>>>>> mutex,
>>>>> making it so that when the primary CPU tries to acquire it to issue
>>>>> the
>>>>> write it hangs. Is there something that I am misunderstanding here?
>>>>>
>>>
>>>> It's my understanding that using the Regmap wrappers _prevents_ locking
>>>> issues, rather than causes them.
>>>
>>> In the case where the CPU is being powered off during a regmap write
>>> there is a potential issue - as Diogo says if we're in the middle of
>>> holding the lock and we power off the CPU that owns the lock then it
>>> will never be able to release the lock.  I would expect the same issue
>>> to apply to a bus like I2C or SPI though, they'll hold a lock while
>>> they're in the middle of doing bus I/O unless you use some special API.
>>
>> Sounds bad
>>
>> Diogo, check if shutdown works with added nosmp to kernel's cmdline.
> 
> So to be clear shutdown already works with regmap_update_bits() and I
> have never encountered this deadlock in my testing as the write to power
> off the PMIC needs to happen at a very specific timing. I imagine adding
> nosmp will just guarantee that the deadlock can never happen.
> 
>> BTW, you can use i2c_smbus_read_byte_data+i2c_smbus_write_byte_data to
>> keep the old regmap_update_bits behaviour.
> 
> My question here is more if this is actually needed or we can skip the
> read. In any case the patch that Lee merged is with regmap_update_bits()
> so for the time being this is not a problem.
I'd suggest not to change anything if there is no real problem.
Otherwise you pleasing ai bot without understanding the problem, maybe
problem doesn't exist in practice.