linux-kernel - Re: [PATCH v1 2/2] drivers/gpu/drm: don't select DMA

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Date:   Thu, 8 Apr 2021 12:44:46 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Joel Stanley <joel@....id.au>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Jeffery <andrew@...id.au>,
        Lucas Stach <l.stach@...gutronix.de>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Michal Simek <michal.simek@...inx.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Peter Collingbourne <pcc@...gle.com>,
        linux-aspeed <linux-aspeed@...ts.ozlabs.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        The etnaviv authors <etnaviv@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>
Subject: 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 12:29 PM David Hildenbrand <david@...hat.com> wrote:
> On 08.04.21 12:20, Arnd Bergmann wrote:
> > On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@...hat.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.
>
> I thought that was the case with "select" :)

Yes, but that's a different set of problems

> >
> > In particular, it does not prevent a configuration with 'DRM_CMA=m'
>
> I assume you meant "DRM_CMA=n" ? DRM_CMA cannot be built as a module.

Ok, at least that makes it easier.

> > and 'DRMA_ASPEED_GFX=y', or any build failures from such
> > a configuration.
>
> I don't follow. "DRM_CMA=n" and 'DRMA_ASPEED_GFX=y' is supposed to work
> just fine (e.g., without HAVE_DMA_CONTIGUOUS) or what am I missing?

I thought you were trying to solve the problem where DRMA_ASPEED_GFX
can optionally link against CMA but would fail to build when the CMA code
is in a loadable module.

If the problem you are trying to solve is a different one, you need a different
solution, not what I posted above.

> > If you want this kind of soft dependency, you need
> > 'depends on DRM_CMA || !DRM_CMA'.
>
> Seriously? I think the point of imply is "please enable if possible and
> not prevented by someone else".

That used to be the meaning, but it changed a few years ago. Now
it means "when a used manually turns on this symbol, turn on the
implied one as well, but let them turn it off again if they choose".

This is pretty much a NOP.

> Your example looks more like a NOP - no?
> Or will it have the same effect?

The example I gave is only meaningful if both are tristate, which is
not the case here as you explain.

It is a somewhat awkward way to say "prevent this symbol from
being =y if the dependency is =m".

      Arnd