Re: [PATCH -next] fbdev: offb: allow build when DRM_OFDRM=m

[Randy Dunlap <rdunlap@infradead.org>]

On 11/23/22 02:02, Randy Dunlap wrote:
> Hi Arnd,
> 
> On 11/23/22 01:08, Arnd Bergmann wrote:
>> On Wed, Nov 23, 2022, at 04:16, Randy Dunlap wrote:
>>> Fix build when CONFIG_FB_OF=y and CONFIG_DRM_OFDRM=m.
>>> When the latter symbol is =m, kconfig downgrades (limits) the 'select's
>>> under FB_OF to modular (=m). This causes undefined symbol references:
>>>
>>> powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x58): 
>>> undefined reference to `cfb_fillrect'
>>> powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x60): 
>>> undefined reference to `cfb_copyarea'
>>> powerpc64-linux-ld: drivers/video/fbdev/offb.o:(.data.rel.ro+0x68): 
>>> undefined reference to `cfb_imageblit'
>>>
>>> Fix this by allowing FB_OF any time that DRM_OFDRM != y so that the
>>> selected FB_CFB_* symbols will become =y instead of =m.
>>>
>>> In tristate logic (for DRM_OFDRM), this changes the dependency from
>>>     !DRM_OFDRM	== 2 - 1 == 1 => modular only (or disabled)
>>> to (boolean)
>>>     DRM_OFDRM != y == y, allowing the 'select's to cause the
>>> FB_CFB_* symbols to =y instead of =m.
>>>
>>
>> Is it actually a useful configuration to have OFDRM=m and
>> FB_OF=y though? I would expect in that case that the OFDRM
>> driver never binds to a device because it's already owned
>> by FB_OF.
>>
>>> diff -- a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
>>> --- a/drivers/video/fbdev/Kconfig
>>> +++ b/drivers/video/fbdev/Kconfig
>>> @@ -455,7 +455,7 @@ config FB_ATARI
>>>  config FB_OF
>>>  	bool "Open Firmware frame buffer device support"
>>>  	depends on (FB = y) && PPC && (!PPC_PSERIES || PCI)
>>> -	depends on !DRM_OFDRM
>>> +	depends on DRM_OFDRM != y
>>>  	select APERTURE_HELPERS
>>
>> I would instead make this 'depends on DRM_OFDRM=n', which
>> completely eliminates configs that have both driver enabled.
> 
> Yep, that works for me. Thanks.
> 
> Thomas, Michal, are you OK with that change?
> 
>> A nicer change would be to make FB_OF a tristate symbol,
>> which makes it possible to load one of the two modules if
>> both are enabled =m, while only allowing one of them to
>> be =y if the other is completely disabled. It looks like
>> offb was originally written to be usable as a loadable module,
>> but Kconfig has prevented this since at least the start of
>> the git history.

ISTM that a distro would prefer to have both DFM_OFDRM and
FB_OF as tristate symbols that could both be built as loadable
modules, as Arnd describes above.
I'll look into that.

-- 
~Randy