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

[Jocelyn Falempe <jfalempe@redhat.com>]

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.

Best regards,

-- 

Jocelyn

> 
>>
>> By other words, does it mean that "dlog->suspended == true" is set
>> only when CON_SUSPENDED is already set in the related con->flags?
>> 8
>>> Just make sure we don't write to the framebuffer while the graphic
>>> driver is suspended.
>>> It may lose a few messages between graphic suspend and console
>>> suspend.
>>
>> The messages should not get lost when the console is properly
>> suspended by suspend_console(), set CON_SUSPENDED.
>>
>> Or maybe, I do not understand it correctly. Maybe you want to say
>> that it should work correctly even without this patch. And this
>> patch creates just a safeguard to make sure that nothing wrong
>> happens even when suspend_console() was not called from some
>> reasons.
> 
> I mean that with this patch if the console is suspended after the 
> graphic driver, then the message between the suspend of the graphic 
> driver and the suspend of the console won't be drawn. I don't see that 
> as a big problem, if you debug suspend/resume with drm_log, and the 
> screen goes blank, you won't see much anyway. And using dmesg when the 
> system is resumed, would have all the messages.
> 
> Without this patch, it may crash if the framebuffer is no more 
> accessible, and drm_log tries to draw a new line on it.
>>
>>
>> Note: I tried to check the order by reading the code. But
>>        drm_log_client_suspend() was called via too many layers.
>>        And I was not able to find where exactly it was called,
>>        for example, from hibernate() in kernel/power/hibernate.c
>>
>>
>>> --- a/drivers/gpu/drm/drm_log.c
>>> +++ b/drivers/gpu/drm/drm_log.c
>>> @@ -310,10 +311,32 @@ static int drm_log_client_hotplug(struct 
>>> drm_client_dev *client)
>>>       return 0;
>>>   }
>>> +static int drm_log_client_suspend(struct drm_client_dev *client, 
>>> bool _console_lock)
>>> +{
>>> +    struct drm_log *dlog = client_to_drm_log(client);
>>> +
>>> +    mutex_lock(&dlog->lock);
>>> +    dlog->suspended = true;
>>> +    mutex_unlock(&dlog->lock);
>>
>> It might also be possible to explicitly set the CON_SUSPENDED flag
>> here to be always on the safe side. We could create variant of
>> suspend_console() just for one console. Something like:
>>
>> void suspend_one_console(struct console *con)
>> {
>>     struct console *con;
>>
>>     if (!console_suspend_enabled)
>>         return;
>>
>>     pr_info("Suspending console(%s) (use no_console_suspend to 
>> debug)\n");
>>     pr_flush(1000, true);
>>
>>     console_list_lock();
>>     if (con && console_is_registered_locked(con))
>>         console_srcu_write_flags(con, con->flags | CON_SUSPENDED);
>>     console_list_unlock();
>>
>>     /*
>>      * Ensure that all SRCU list walks have completed. All printing
>>      * contexts must be able to see that they are suspended so that it
>>      * is guaranteed that all printing has stopped when this function
>>      * completes.
>>      */
>>     synchronize_srcu(&console_srcu);
>> }
>>
>> and call here:
>>
>>     suspend_one_console(dlog->con);
>>
>>
>> But this is not needed when the console is already supposed to be
>> disabled here. If this is the case then it might be better
>> to just check and warn when it does not happen. Something like:
>>
>> void assert_console_suspended(struct console *con)
>> {
>>     int cookie;
>>
>>     cookie = console_srcu_read_lock();
>>
>>     /* Do not care about unregistered console */
>>     if (!con || hlist_unhashed_lockless(&con->node))
>>         goto out;
>>
>>     if (WARN_ON_ONCE(!(console_srcu_read_flags(con) & CON_SUSPENDED)))
>>         pr_flush(1000, true);
>> out:
>>     console_srcu_read_unlock(cookie);
>> }
>>
>>> +    return 0;
>>> +}
>>
>>
> 
> Thanks for this two suggestions, this is really what I was looking for.
> I will run some tests on real hardware, to see which one is suspended 
> first.
> 
> Best regards,
>