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Message-ID: <CAK8P3a09LdJ-87ZrN28y=t8Sa0zL-3NOvEWhkStMY+2EbO7UAw@mail.gmail.com>
Date: Thu, 8 Apr 2021 12:20:50 +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 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.
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
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