linux-kernel - Re: [PATCH] drm/tiny: pixpaper: add explicit dependency on MMU

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Message-ID: <bd93498e-749c-4e85-9c63-918aa8f2d6f3@suse.de>
Date: Wed, 29 Oct 2025 10:49:22 +0100
From: Thomas Zimmermann <tzimmermann@...e.de>
To: LiangCheng Wang <zaq14760@...il.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] drm/tiny: pixpaper: add explicit dependency on MMU

Hi

Am 28.10.25 um 03:55 schrieb LiangCheng Wang:
> The DRM_GEM_SHMEM_HELPER helper requires MMU enabled because it uses
> vmf_insert_pfn() in its mmap implementation. On NOMMU configurations
> (e.g. some RISC-V randconfig builds), this symbol is unavailable and
> selecting DRM_GEM_SHMEM_HELPER causes a modpost undefined reference:
>
>      ERROR: modpost: "vmf_insert_pfn" [drivers/gpu/drm/drm_shmem_helper.ko] undefined!
>
> Normally, Kconfig prevents this helper from being selected when
> CONFIG_MMU=n. However, in some randconfig builds (such as those used by
> 0day CI), select statements can override unmet dependencies, triggering
> the issue.
>
> Add an explicit dependency on MMU to DRM_PIXPAPER to prevent this.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202510280213.0rlYA4T3-lkp@intel.com/
> Fixes: 0c4932f6ddf8 ("drm/tiny: pixpaper: Fix missing dependency on DRM_GEM_SHMEM_HELPER")
> Signed-off-by: LiangCheng Wang <zaq14760@...il.com>
> ---
> This patch fixes a build issue reported by the kernel test robot when
> building with CONFIG_MMU=n on RISC-V randconfig.
>
> In such configurations, the DRM_GEM_SHMEM_HELPER helper uses
> vmf_insert_pfn(), which is unavailable without MMU support. Although
> normal Kconfig rules prevent DRM_GEM_SHMEM_HELPER from being selected
> in this case, randconfig builds may forcibly enable it via select,
> leading to the following modpost error:
>
>      ERROR: modpost: "vmf_insert_pfn" [drivers/gpu/drm/drm_shmem_helper.ko] undefined!
>
> The fix is to add an explicit `depends on MMU` to the DRM_PIXPAPER
> driver, ensuring it cannot be built for NOMMU systems.
>
> This issue cannot always be reproduced locally because typical builds
> do not violate Kconfig dependencies, but it was confirmed that the fix
> prevents the reported 0day failure.

The problem most likely comes from [1]. Is it possible to fix this instead?

[1] https://elixir.bootlin.com/linux/v6.17.5/source/arch/riscv/Kconfig#L1345

Best regards
Thomas

> ---
> LiangCheng Wang (1):
>    drm/tiny: pixpaper: add explicit dependency on MMU
>
>   drivers/gpu/drm/tiny/Kconfig
>
> Signed-off-by: LiangCheng Wang <zaq14760@...il.com>
> ---
>   drivers/gpu/drm/tiny/Kconfig | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/gpu/drm/tiny/Kconfig b/drivers/gpu/drm/tiny/Kconfig
> index 7d9e85e932d7fd7bdb6ad7a4c6ba0f835841f623..f0e72d4b6a4709564e63c758e857bdb4a320dbe7 100644
> --- a/drivers/gpu/drm/tiny/Kconfig
> +++ b/drivers/gpu/drm/tiny/Kconfig
> @@ -85,6 +85,7 @@ config DRM_PANEL_MIPI_DBI
>   config DRM_PIXPAPER
>           tristate "DRM support for PIXPAPER display panels"
>           depends on DRM && SPI
> +        depends on MMU
>           select DRM_CLIENT_SELECTION
>           select DRM_GEM_SHMEM_HELPER
>           select DRM_KMS_HELPER
>
> ---
> base-commit: fd57572253bc356330dbe5b233c2e1d8426c66fd
> change-id: 20251028-bar-ae0d85b16f13
>
> Best regards,

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)