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

[David Hildenbrand]

On 08.04.21 22:29, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 6:45 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 08.04.21 14:49, Linus Walleij wrote:
> > > On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > > > 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 ...
> >
> > > 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.

Right, and even at runtime there is no guarantee that DMA_CMA will do 
anything -- especially if we don't reserve a CMA region (e.g., "cma=X").

So this really sounds like a

"desires DMA_CMA"

and achieving that via an additional symbol or via "default y if ..." 
for DMA_CMA sounds reasonable.

Thanks!

--
Thanks,

David / dhildenb