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Subject: Re: gpu: drm_fb_cma_helper.c:46: undefined reference to
 `drm_gem_fb_get_obj'
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        open list <linux-kernel@vger.kernel.org>,
        Linux-Next Mailing List <linux-next@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Andrey Konovalov <andreyknvl@gmail.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Linus Walleij <linus.walleij@linaro.org>
References: <CA+G9fYvpyUbqLko+9Dza8h4=9yOd-n9J0dKoQtZxawstCCnsZw@mail.gmail.com>
 <857ab1a9-0175-2b2c-e729-2620d0221e1e@suse.de>
 <6862b109-ea12-6ffa-c82b-b23ee26aa5b2@infradead.org>
 <CAK8P3a0wG8dKnuQMOL=bKmBHuSkWcu6OfvhTP-86rpLdr7_5CA@mail.gmail.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <61f14f2b-b1cd-b9df-86fd-8fcc4b9eb738@infradead.org>
Date:   Tue, 26 Oct 2021 23:26:07 -0700
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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