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

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Date:   Thu, 8 Apr 2021 23:45:23 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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>,
        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 Thu, Apr 8, 2021 at 6:44 PM David Hildenbrand <david@...hat.com> wrote:

> > drivers/gpu/drm/mcde/Kconfig
> > drivers/gpu/drm/pl111/Kconfig
> > drivers/gpu/drm/tve200/Kconfig
>
> I was assuming these are "real" dependencies. Will it also work without
> DMA_CMA?

It will mostly work but that is only because the reservations are
mostly contiguous anyway because they are done early and
are small. The hardware requires contiguous buffers in all
three cases. I'm not sure I always got it right.

> > certainly needs this as well, and pretty much anything that is
> > selecting DRM_KMS_CMA_HELPER or
> > DRM_GEM_CMA_HELPER "wants" DMA_CMA.
>
> "wants" as in "desires to use but can life without" or "wants" as in
> "really needs it". ?

I don't know the exact semantics of using DRM_KMS_CMA*
without actually using DMA_CMA. I suspect small allocations
will be contiguous and big allocations will start to fragment?
but it's just my guess. I guess "really need it"?

Yours,
Linus Walleij