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Subject: Re: [PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from
 aspeed or etnaviv
From:   David Hildenbrand <david@redhat.com>
To:     Arnd Bergmann <arnd@arndb.de>
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>
References: <20210408092011.52763-1-david@redhat.com>
 <20210408092011.52763-3-david@redhat.com>
 <CAK8P3a09LdJ-87ZrN28y=t8Sa0zL-3NOvEWhkStMY+2EbO7UAw@mail.gmail.com>
 <cd14d4b4-da82-b21c-2cd6-8e474d97b955@redhat.com>
Organization: Red Hat GmbH
Message-ID: <0b1928a8-99c3-f1f1-ad66-40145199d9bc@redhat.com>
Date:   Thu, 8 Apr 2021 12:36:59 +0200
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On 08.04.21 12:27, David Hildenbrand wrote:
> On 08.04.21 12:20, Arnd Bergmann wrote:
>> 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.
> 
> I thought that was the case with "select" :)
> 
>>
>> 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.
> 
>> 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?
> 
>>
>> 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". Your example looks more like a NOP - no?
> Or will it have the same effect?

I just tried (remove CONFIG_DMA_CMA from .config followed by make) and 
the default will be set to "N" (when querying the user). So it indeed 
looks like a NOP - unless I am missing something.

-- 
Thanks,

David / dhildenb