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

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Message-ID: <3a2d64a7-8425-8daf-17ee-95b9f0c635f9@redhat.com>
Date:   Thu, 8 Apr 2021 14:00:48 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
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 08.04.21 13:44, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 1:00 PM David Hildenbrand <david@...hat.com> wrote:
>>>
>>> It is a somewhat awkward way to say "prevent this symbol from
>>> being =y if the dependency is =m".
>>
>> What would be the right thing to do in the case here then to achieve the
>> "if DRMA_ASPEED_GFX is enabled, also enable DMA_CMA id possible"?
>>
>> One approach could be to have for DMA_CMA
>>
>> default y if DRMA_ASPEED_GFX
>>
>> but it feels like the wrong way to tackle this.
> 
> I'm still not sure what you are trying to achieve. Is the idea only to provide
> a useful default for DMA_CMA depending on which drivers are enabled?

"Random drivers should not override a user configuration of core knobs
(e.g., CONFIG_DMA_CMA=n)."

Let's assume I'm a distribution and want to set CONFIG_CMA=n or want to 
set CONFIG_DMA_CMA=n with CONFIG_CMA=y; there is no way to do that with 
e.g., DRMA_ASPEED_GFX=y because it will always override my (user!) 
setting -- even though it doesn't really always need it. Using "select" 
is the problem here.

> 
> 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 !) 
I wondered if there is a better approach to achieve the same; "imply" 
sounded like a good candidate.

-- 
Thanks,

David / dhildenb