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

[Jocelyn Falempe <jfalempe@redhat.com>]

On 01/11/2024 17:00, Petr Mladek wrote:
> On Fri 2024-10-25 11:46:16, Jocelyn Falempe wrote:
>> On 25/10/2024 01:11, Jocelyn Falempe wrote:
>>> On 24/10/2024 16:34, Petr Mladek wrote:
>>>> On Wed 2024-10-23 14:00:13, Jocelyn Falempe wrote:
>>>>> The console is already suspended in printk.c.
>>>>
>>>> Does this mean that drm_log_client_suspend() is called
>>>> after suspend_console(), please?
>>>
>>> To be honest, I wasn't able to tell which one is called first, and if
>>> the order is enforced (I didn't check if drivers can be suspended in
>>> parallel, or if it's all sequential)..
>>>
>>> I then checked if it's possible to suspend the console, but didn't found
>>> an easy API to do so, so I went with this lazy patch, just ensuring
>>> we're not writing to a suspended graphic driver.
>>
>> I've run some tests on my hardware, and the console is suspended before the
>> graphic driver:
>>
>> [   56.409604] printk: Suspending console(s) (use no_console_suspend to
>> debug)
>> [   56.411430] serial 00:05: disabled
>> [   56.421877] sd 0:0:0:0: [sda] Synchronizing SCSI cache
>> [   56.421954] sd 4:0:0:0: [sdb] Synchronizing SCSI cache
>> [   56.422545] ata1.00: Entering standby power mode
>> [   56.422793] DRM log suspend
>>
>> But because there is the "no_console_suspend" parameter, and we should make
>> sure to not draw when the graphic driver is suspended, I think this patch is
>> needed, and good enough.
>> I will just rephrase the commit message, to make it clear, that some message
>> won't be drawn, only if "no_console_suspend" is set.
> 
> Ah, I forgot about the "no_console_suspend" parameter. The problem
> with this patch is that it would quietly drop all pending messages.
> 
> drm_log_write_thread() does not have any return value.
> nbcon_emit_next_record() would assume that the message was printed.
> And the kthread would continue emitting next message...
> 
> In compare, CON_SUSPENDED would cause that console_is_usable()
> returns false. As a result, nbcon_kthread_func() would not try
> to emit any message and go into a sleep.
> 
> If we set CON_SUSPENDED then the pending messages will get printed
> after the resume. If we use this patch, the messages would get lost.
> 
> 
> This is why I am not happy with this patch. I would prefer to
> block the console. I see three better solutions:
> 
>    1. Set CON_SUSPENDED from drm_log_client_suspend even when
>       "no_console_suspend" is used.
> 
>       It is a bit dirty and might cause some confusion.
> 
> 
>    2. Add a new flag, e.g. CON_BLOCKED or CON_DRIVER_BLOCKED,
>       which might be used for this purpose.
> 
> 
>    3. Allow con->write_thread() to return an error code.
> 
>       The question is how exactly the error should be handled.
>       The kthread would not know when the printing might succeed
>       again.
> 
> 
> I personally prefer the 2nd variant.

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 ?

https://elixir.bootlin.com/linux/v6.11.6/source/drivers/tty/serial/serial_core.c#L2462

https://elixir.bootlin.com/linux/v6.11.6/source/kernel/printk/printk.c#L3323

It looks like it should do exactly what we need ?

Best regards,

-- 

Jocelyn

> 
> 
> Best Regards,
> Petr
> 
> PS: I am sorry for the late reply. I had vacation...
>