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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 8 Apr 2021 12:20:50 +0200
Message-ID: <CAK8P3a09LdJ-87ZrN28y=t8Sa0zL-3NOvEWhkStMY+2EbO7UAw@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from
 aspeed or etnaviv
To:     David Hildenbrand <david@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>, Joel Stanley <joel@jms.id.au>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Andrew Jeffery <andrew@aj.id.au>,
        Lucas Stach <l.stach@pengutronix.de>,
        Russell King <linux+etnaviv@armlinux.org.uk>,
        Christian Gmeiner <christian.gmeiner@gmail.com>,
        Mike Rapoport <rppt@kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Michal Simek <michal.simek@xilinx.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Peter Collingbourne <pcc@google.com>,
        linux-aspeed <linux-aspeed@lists.ozlabs.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        The etnaviv authors <etnaviv@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>
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On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.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