Re: [PATCH v5 5/6] drm/log: Implement suspend/resume

[Jocelyn Falempe <jfalempe@redhat.com>]

On 04/11/2024 15:15, Petr Mladek wrote:
> On Mon 2024-11-04 11:52:33, John Ogness wrote:
>> On 2024-11-04, Jocelyn Falempe <jfalempe@redhat.com> wrote:
>>> I looked at what serial drivers are doing, because they can also have
>>> their clock gated in suspend.
>>>
>>> Would calling console_stop() in the suspend and console_start() in
>>> resume work ?
>>
>> Yes. That is what it is for.
> 
> It seems that you are right. I have never really investigated the purpose
> of this API /o\
> 
Thanks, I will send a v6 with that change.

> One problem with this API is that it does not check whether the
> console is registered. I wonder whether it might cause problems.

At least for drm_log, register_console() will always be called before.
> 
> For example, we should not set the CON_ENABLE flag when the console is not
> registered. Doing so would cause register_console() to always enable
> the console, even when it is not preferred.
> 
> Additionally, nbcon_kthread_wake() uses con->rcuwait, which is initialized
> by nbcon_alloc() called from register_console(). Fortunately, nbcon_alloc()
> is always called even if the console is not enabled in the end, but this
> might change in the future and cause subtle errors.
> 
> [ After even more thinking ]
> 
> I wonder whether console_start()/console_stop() should really
> manipulate CON_ENABLE flag. It might be historical solution when
> @console_suspended was a global variable.
> 
> But it has changed with the commit 9e70a5e109a4a2336 ("printk: Add
> per-console suspended state").
> 
> It might make more sense when console_start()/console_stop()
> manipulates CON_SUSPENDED flag. Then it would make sense
> to rename them suspend_this_console()/resume_this_console().
> 
> What do you think?

Maybe when registering the console, having a flag to say "I want this 
console to be suspended with the console subsystem" or "I want to handle 
suspend/resume on my own, and call the relevant functions" would be better ?

That would avoid having the same console being suspended/resumed twice, 
and making clear what to expect.

Of course "no_console_suspend" won't really work for drivers handling 
suspend/resume themselves.

-- 

Jocelyn
> 
> Best Regards,
> Petr
>