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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Fri, 9 Apr 2021 14:35:52 +0200
Message-ID: <CACRpkdb_eQrcCFV9Q9uho8E6HO4X0dRJp1qAdJBcT82=sZ1v7A@mail.gmail.com>
Subject: Re: [PATCH v3] drivers: introduce and use WANT_DMA_CMA for soft
 dependencies on DMA_CMA
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
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On Fri, Apr 9, 2021 at 1:20 PM David Hildenbrand <david@redhat.com> wrote:

> Random drivers should not override a user configuration of core knobs
> (e.g., CONFIG_DMA_CMA=n). Applicable drivers would like to use DMA_CMA,
> which depends on CMA, if possible; however, these drivers also have to
> tolerate if DMA_CMA is not available/functioning, for example, if no CMA
> area for DMA_CMA use has been setup via "cma=X". In the worst case, the
> driver cannot do it's job properly in some configurations.

Looks good to me. At least a lot better than what we have.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>

> Let's see if this approach is better for soft dependencies (and if we
> actually have some hard dependencies in there). This is the follow-up
> of
>   https://lkml.kernel.org/r/20210408092011.52763-1-david@redhat.com
>   https://lkml.kernel.org/r/20210408100523.63356-1-david@redhat.com

You can just add these to the commit message with Link:
when applying so people can easily find the discussion from the
commit.

> I was wondering if it would make sense in some drivers to warn if either
> CONFIG_DMA_CMA is not available or if DRM_CMA has not been configured
> properly - just to give people a heads up that something might more likely
> go wrong; that would, however, be future work.

I think the frameworks  (DRM_*_CMA_HELPER)
should pr_info something about it so the individual drivers
don't have to sanity check their entire world.

Yours,
Linus Walleij