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

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Date:   Thu, 8 Apr 2021 13:38:17 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     David Hildenbrand <david@...hat.com>,
        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 08, 2021 at 12:20:50PM +0200, 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.
> 
> 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'.

The problem is that depends on is a real pain for users to find their
drivers. This is why we have a ton of select, because the kconfig ui tends
to suck.

If you want to change this, we need automatic conflict resolution like apt
and other package managers have, with suggestions how to fix the config if
you want to enable a driver, but some of its requirements are missing. The
current approach of hiding driver symbols complete if any of their
dependencies are off is really not great.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch