Re: [PATCH v2] serial: stm32: optimize spin lock usage

[Johan Hovold <johan@kernel.org>]

[ Please avoid top-posting. ]

On Thu, Apr 15, 2021 at 07:09:14PM +0200, Erwan LE RAY wrote:
> Hi Dillon,
> 
> STM32MP151 is mono-core, but both STM32MP153 and STM32MP157 are 
> dual-core (see 
> https://www.st.com/content/st_com/en/products/microcontrollers-microprocessors/stm32-arm-cortex-mpus.html).
> So your point is fully relevant, thanks.
> 
> ST already fixed the same issue in st-asc.c driver in the past (see 
> ef49ffd8), because a systematic deadlock was detected with RT kernel.

That's not the same issue. The above mentioned commit fixed an issue on
*RT* where local_irq_save() should be avoided.

> You proposed a first implementation in your patch, and a second one in 
> the discussion. It seems that your initial proposal (ie your V2 patch) 
> is the most standard one (implemented in 6 drivers). The second 
> implementation is implemented by only 1 company.
> 
> It looks that the solution is to avoid locking in the sysrq case and 
> trylock in the oops_in_progress case (see detailed analysis in 
> 677fe555cbfb1).
>
> So your initial patch looks to the right proposal, but it would be safer 
> if Greg could confirm it.

That would only fix the RT issue (and by making the sysrq one slightly
worse).

Using uart_unlock_and_check_sysrq() would address both issues.

Johan