linux-kernel - Re: Alternative approach to solve the deferred probe

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Message-ID: <5627C381.60602@ti.com>
Date:	Wed, 21 Oct 2015 19:55:29 +0300
From:	Grygorii Strashko <grygorii.strashko@...com>
To:	<frowand.list@...il.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
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Subject: Re: Alternative approach to solve the deferred probe

On 10/21/2015 06:36 PM, Frank Rowand wrote:
> On 10/21/2015 1:18 AM, Russell King - ARM Linux wrote:
>> On Tue, Oct 20, 2015 at 08:58:19PM -0700, Frank Rowand wrote:
>>> On 10/20/2015 8:46 AM, Russell King - ARM Linux wrote:
> 
> < snip >
> 
>>>> +
>>>>   static bool driver_deferred_probe_enable = false;
>>>> +
>>>>   /**
>>>>    * driver_deferred_probe_trigger() - Kick off re-probing deferred devices
>>>>    *
>>>> @@ -188,6 +210,13 @@ static int deferred_probe_initcall(void)
>>>>   	driver_deferred_probe_trigger();
>>>
>>> Couldn't you put the "driver_deferred_probe_report = true" here?  And then
>>> not add another round of probes.
>>
>> The idea is not to report anything for drivers that were deferred
>> during the normal bootup.  The above is part of the normal bootup,
>> and the deferred activity should not be warned about.
> 
> The above is currently the last point for probe to succeed or defer
> (until possibly, as you mentioned, module loading resolves the defer).
> If a probe defers above, it will defer again below.  The set of defers
> should be exactly the same above and below.
> 

Unfortunately this is not "the last point for probe to succeed or defer".
There are still a bunch of drivers in Kernel which will be probed at late_initcall() level.
(like ./drivers/net/ethernet/ti/cpsw.c => late_initcall(cpsw_init);
Yes - they probably need to be updated to use module_init(), but that's what
we have now). Those drivers will re-trigger deferred device probing if their
probe succeeded.

As result, it is impossible to say when will it happen the 
"final round of deferred device probing" :( and final list of drivers which
was "deferred forever" will be know only when kernel exits to User space 
("deferred forever" - before loading modules).

May be, we also can consider adding debug_fs entry which can be used to display
actual state of deferred_probe_pending_list? 

>>
>> If we have any devices still deferring after _this_ round, that must
>> indicate that some resource they want is not available, and that
>> should be warned about.
>>
>> Of course, modules can defer too - and I made some suggestions in my
>> waffle above the patch about that.
>>
> 
> < adding back trimmed, for fuller context >
> 
>>>>   	/* Sort as many dependencies as possible before exiting initcalls */
>>>>   	flush_workqueue(deferred_wq);
>>>> +
>>>> +	/* Now one final round, reporting any devices that remain deferred */
>>>> +	driver_deferred_probe_report = true;
>>>> +	driver_deferred_probe_trigger();
>>>> +	/* Sort as many dependencies as possible before exiting initcalls */
>>>> +	flush_workqueue(deferred_wq);
>>>> +
>>>>   	return 0;
>>>>   }


-- 
regards,
-grygorii
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