Re: [PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann <arnd@arndb.de>]

On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.com> wrote:
>
> Random drivers should not override a user configuration of core knobs
> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
> dependencies and manual overrides.
>
> "This is similar to "select" as it enforces a lower limit on another
>  symbol except that the "implied" symbol's value may still be set to n
>  from a direct dependency or with a visible prompt."
>
> Implying DRM_CMA should be sufficient, as that depends on CMA.
>
> Note: If this is a real dependency, we should use "depends on DMA_CMA"
> instead -  but I assume the driver can work without CMA just fine --
> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.

'imply' is almost never the right solution, and it tends to cause more
problems than it solves.

In particular, it does not prevent a configuration with 'DRM_CMA=m'
and 'DRMA_ASPEED_GFX=y', or any build failures from such
a configuration.

If you want this kind of soft dependency, you need
'depends on DRM_CMA || !DRM_CMA'.

         Arnd