Re: [PATCH] drm/panthor: Fix the CONFIG_PM=n case

[Steven Price <steven.price@arm.com>]

On 18/03/2024 13:08, Boris Brezillon wrote:
> On Mon, 18 Mar 2024 11:31:05 +0000
> Steven Price <steven.price@arm.com> wrote:
> 
>> On 18/03/2024 08:58, Boris Brezillon wrote:
>>> Putting a hard dependency on CONFIG_PM is not possible because of a
>>> circular dependency issue, and it's actually not desirable either. In
>>> order to support this use case, we forcibly resume at init time, and
>>> suspend at unplug time.
>>>
>>> Reported-by: kernel test robot <lkp@intel.com>
>>> Closes: https://lore.kernel.org/oe-kbuild-all/202403031944.EOimQ8WK-lkp@intel.com/
>>> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>  
>>
>> Reviewed-by: Steven Price <steven.price@arm.com>
>>
>>> ---
>>> Tested by faking CONFIG_PM=n in the driver (basically commenting
>>> all pm_runtime calls, and making the panthor_device_suspend/resume()
>>> calls unconditional in the panthor_device_unplug/init() path) since
>>> CONFIG_ARCH_ROCKCHIP selects CONFIG_PM. Seems to work fine, but I
>>> can't be 100% sure this will work correctly on a platform that has
>>> CONFIG_PM=n.  
>>
>> The same - I can't test this properly :(
>>
>> Note that the other option (which AFAICT doesn't cause any problems) is
>> to "select PM" rather than depend on it - AIUI the 'select' dependency
>> is considered in the opposite direction by kconfig so won't cause the
>> dependency loop.
> 
> Doesn't seem to work with COMPILE_TEST though? I mean, we need
> something like
> 
> 	depends on ARM || ARM64 || (COMPILE_TEST && PM)
> 	...
> 	select PM
> 
> but kconfig doesn't like that

Why do we need the "&& PM" part? Just:

	depends on ARM || ARM64 || COMPILE_TEST
	...
	select PM

Or at least that appears to work for me.

> drivers/gpu/drm/panthor/Kconfig:3:error: recursive dependency detected!
> drivers/gpu/drm/panthor/Kconfig:3:	symbol DRM_PANTHOR depends on
> PM kernel/power/Kconfig:183:	symbol PM is selected by DRM_PANTHOR
> 
> which id why I initially when for a depends on PM
> 
> 
>> Of course if there is actually anyone who has a
>> platform which can be built !CONFIG_PM then that won't help. But the
>> inability of anyone to actually properly test this configuration does
>> worry me a little.
> 
> Well, as long as it doesn't regress the PM behavior, I think I'm happy
> to take the risk. Worst case scenario, someone complains that this is
> not working properly when they do the !PM bringup :-).

Indeed, I've no objection to this patch - although I really should have
compiled tested it as Robin pointed out ;)

But one other thing I've noticed when compile testing it - we don't
appear to have fully fixed the virt_to_pfn() problem. On x86 with
COMPILE_TEST I still get an error. Looking at the code it appears that
virt_to_pfn() isn't available on x86... it overrides asm/page.h and
doesn't provide a definition. The definition on x86 is hiding in
asm/xen/page.h.

Outside of arch code it's only drivers/xen that currently uses that
function. So I guess it's probably best to do a
PFN_DOWN(virt_to_phys(...)) instead. Or look to fix x86 :)

Steve