Re: gpu: drm_fb_cma_helper.c:46: undefined reference to `drm_gem_fb_get_obj'

[Randy Dunlap <rdunlap@infradead.org>]

On 10/26/21 11:19 PM, Arnd Bergmann wrote:
> On Wed, Oct 27, 2021 at 2:58 AM Randy Dunlap <rdunlap@infradead.org> wrote:
> 
>>>
>>> Looking at this config, there is:
>>>
>>> CONFIG_DRM=y
>>> # CONFIG_DRM_DP_AUX_CHARDEV is not set
>>> # CONFIG_DRM_DEBUG_MM is not set
>>> # CONFIG_DRM_DEBUG_SELFTEST is not set
>>> CONFIG_DRM_KMS_HELPER=m
>>> # CONFIG_DRM_LOAD_EDID_FIRMWARE is not set
>>> # CONFIG_DRM_DP_CEC is not set
>>> CONFIG_DRM_GEM_CMA_HELPER=y
>>> CONFIG_DRM_KMS_CMA_HELPER=y
>>>
>>> GEM_CMA_HELPER depends on KMS_HELPER, but the latter is a module. That's probably the cause of the problem. Is it intentionally set this way?
>>>
>>
>> The only drivers that select DRM_KMS_HELPER are both =m, so that's how
>> DRM_KMS_HELPER is set also.
>>
>> Symbol: DRM_KMS_HELPER [=m]
>> Type : tristate
>> Defined at drivers/gpu/drm/Kconfig:82
>> Depends on: HAS_IOMEM [=y] && DRM [=y]
>> Selected by [m]:
>> - DRM_ATMEL_HLCDC [=m] && HAS_IOMEM [=y] && DRM [=y] && OF [=y] && COMMON_CLK [=y] && MFD_ATMEL_HLCDC [=y] && ARM [=y]
>> - DRM_ASPEED_GFX [=m] && HAS_IOMEM [=y] && DRM [=y] && OF [=y] && (COMPILE_TEST [=n] || ARCH_ASPEED [=y]) && MMU [=y]
>>
>>
>> I did the ARM cross-build and also see the linker error.
>> I didn't understand why -- and still don't, but here is a little
>> speculation:
>>
>> In the past (e.g. 10 years ago), we have to move some .o files
>> in lib/ from lib-y to obj-y so that they would always be included
>> in the final object file and not cause their user/caller object
>> files to suffer from undefined references.
>> These happened because unused functions(?) in lib-y files are
>> stripped out of the final object files.
>> The same thing could be happening here (still just guessing).
>>
>> Does that help any?  I dunno.
>>
>> Adding Arnd to Cc: to see if he has any ideas...
> 
>  From all I can tell, the problem is that DRM_KMS_CMA_HELPER is a 'bool'
> symbol, so if it gets selected by a '=m' driver, it turns into '=y', which
> then selects DRM_GEM_CMA_HELPER=y, but that one cannot link
> against DRM_KMS_HELPER=m code.
> 
> This trivial change makes it all build:
> 
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index c08860db2520..699f434ce813 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -218,7 +218,7 @@ config DRM_GEM_CMA_HELPER
>            Choose this if you need the GEM CMA helper functions
> 
>   config DRM_KMS_CMA_HELPER
> -       bool
> +       tristate
>          depends on DRM
>          select DRM_GEM_CMA_HELPER
>          help
> 
> but this needs some more testing to make sure it doesn't add
> any other regressions.
> 
> Interestingly, I never hit the problem in randconfig testing since
> there is always some '=y' driver that selects DRM_KMS_HELPER.
> 
>          Arnd
> 

Hi Arnd,
Thomas had posted a patch that also fixes the problem.
Somehow I missed seeing that the first time around.

Thanks.
-- 
~Randy