Subject: [PATCH] ARM: OMAP2+: omap_device: add fail hook for runtime_pm when bad data is detected

From: [email-masked] (Nishanth Menon)
Date: Wed, 04 Dec 2013 05:33:08 -0600

On 12/04/2013 02:08 AM, Joel Fernandes wrote:
> On 12/04/2013 07:09 AM, Nishanth Menon wrote:
>> Due to the cross dependencies between hwmod for automanaged device
>> information for OMAP and dts node definitions, we can run into scenarios
>> where the dts node is defined, however it's hwmod entry is yet to be
>> added. In these cases:
>> a) omap_device does not register a pm_domain (since it cannot find
>>     hwmod entry).
>> b) driver does not know about (a), does a pm_runtime_get_sync which
>>     never fails
>> c) It then tries to do some operation on the device (such as read the
>>    revision register (as part of probe) without clock or adequate OMAP
>>    generic PM operation performed for enabling the module.
>>
>> This causes a crash such as that reported in:
>> https://bugzilla.kernel.org/show_bug.cgi?id=66441
>>
>> When 'ti,hwmod' is provided in dt node, it is expected that the device
>> will not function without the OMAP's power automanagement. Hence, when
>> we hit a fail condition (due to hwmod entries not present or other
>> similar scenario), fail at pm_domain level due to lack of data, provide
>> enough information for it to be fixed, however, it allows for the driver
>> to take appropriate measures to prevent crash.
>>
>> Reported-by: Tobias Jakobi <[email-masked]>
>> Signed-off-by: Nishanth Menon <[email-masked]>
>> ---
>>   arch/arm/mach-omap2/omap_device.c |   24 ++++++++++++++++++++++++
>>   arch/arm/mach-omap2/omap_device.h |    1 +
>>   2 files changed, 25 insertions(+)
>>
>> diff --git a/arch/arm/mach-omap2/omap_device.c b/arch/arm/mach-omap2/omap_device.c
>> index 53f0735..e0a398c 100644
>> --- a/arch/arm/mach-omap2/omap_device.c
>> +++ b/arch/arm/mach-omap2/omap_device.c
>> @@ -183,6 +183,10 @@ static int omap_device_build_from_dt(struct platform_device *pdev)
>>   odbfd_exit1:
>>   	kfree(hwmods);
>>   odbfd_exit:
>> +	/* if data/we are at fault.. load up a fail handler */
>> +	if (ret)
>> +		pdev->dev.pm_domain = &omap_device_fail_pm_domain;
>> +
>>   	return ret;
>>   }
>>
>
> Just wondering, can't we just print the warning here instead of registering new
> pm_domain callbacks?
>

I suggest you might want to read the commit message again.. but lets try 
once again:

As you see in dmesg log 
https://bugzilla.kernel.org/attachment.cgi?id=117311 pointed in the bug 
https://bugzilla.kernel.org/show_bug.cgi?id=66441,

you already have
"
[    0.176940] platform 4b501000.aes: Cannot lookup hwmod 'aes'
[    0.177215] platform 480a5000.des: Cannot lookup hwmod 'des'"

Now, printing that warning does not help, as I already explained in the 
commit log,
"
 >> b) driver does not know about (a), does a pm_runtime_get_sync which
 >>     never fails"

A device node stated it will have hwmod to adequately control it, but in 
reality, as in this case, it does not. how does printing a warning alone 
help the driver which is not aware of these? The driver's attempt at 
pm_runtime_sync should fail, as that is what "ti,hwmod" property controls.

> Concerned that all this LOC may end up being dead code when the "ti,hwmods"
> property becomes obsolete anyway.

we detected we have a bug with 3.13-rc2 - this is a fix for kernel 
(probably a stable candidate too). ti,hwmod property might become 
eventually obsolete (and we are working towards that), but the 
functionality that it provides today is necessary for the transition 
from mixed dt-hwmod world to pure dt world. - remember we are moving 
from data structure which is used to describe hardware to another which 
again describes hardware in a different form - the kind of bugs we see 
now are expected to be fixed for transition to be smooth for everyone.

without providing adequate warnings, bugs like 
https://bugzilla.kernel.org/show_bug.cgi?id=66441 will need pretty nasty 
debug.

I hope this helps convince you that error code is worth the LoC.

--
Regards,
Nishanth Menon