linux-kernel - Re: [PATCH v3] drivers: introduce and use WANT_DMA_CMA for soft dependencies on DMA

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Date:   Fri, 9 Apr 2021 14:35:52 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        Lucas Stach <l.stach@...gutronix.de>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Eric Anholt <eric@...olt.net>,
        Michal Simek <michal.simek@...inx.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        "Alexander A. Klimov" <grandmaster@...klimov.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Suman Anna <s-anna@...com>, Jason Gunthorpe <jgg@...pe.ca>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        linux-aspeed <linux-aspeed@...ts.ozlabs.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        The etnaviv authors <etnaviv@...ts.freedesktop.org>,
        linux-mips@...r.kernel.org,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>
Subject: Re: [PATCH v3] drivers: introduce and use WANT_DMA_CMA for soft
 dependencies on DMA_CMA

On Fri, Apr 9, 2021 at 1:20 PM David Hildenbrand <david@...hat.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@...aro.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