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

On Thu, Apr 8, 2021 at 6:45 PM David Hildenbrand [off-list ref] wrote:

On 08.04.21 14:49, Linus Walleij wrote:

quoted

On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand [off-list ref] wrote:

quoted

quoted

This is something you could do using a hidden helper symbol like

config DRMA_ASPEED_GFX
         bool "Aspeed display driver"
         select DRM_WANT_CMA

config DRM_WANT_CMA
         bool
         help
            Select this from any driver that benefits from CMA being enabled

config DMA_CMA
         bool "Use CMA helpers for DRM"
         default DRM_WANT_CMA

           Arnd

That's precisely what I had first, with an additional "WANT_CMA" --  but
looking at the number of such existing options (I was able to spot 1 !)

If you do this it probably makes sense to fix a few other drivers
Kconfig in the process. It's not just a problem with your driver.
"my" drivers:

:) I actually wanted to convert them to "depends on DMA_CMA" but ran
into recursive dependencies ...

quoted

drivers/gpu/drm/mcde/Kconfig
drivers/gpu/drm/pl111/Kconfig
drivers/gpu/drm/tve200/Kconfig

Right, this is the main problem caused by using 'select' to
force-enable symbols that other drivers depend on.

Usually, the answer is to be consistent about the use of 'select'
and 'depends on', using the former only to enable symbols that
are hidden, while using 'depends on' for anything that is an
actual build time dependency.

I was assuming these are "real" dependencies. Will it also work without
DMA_CMA?

I think in this case, it is fairly likely to work without DMA_CMA when the
probe function gets called during a fresh boot, but fairly likely to fail if
it gets called after the system has run for long enough to fragment the
free memory.

The point of DMA_CMA is to make it work reliably.

      Arnd

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