linux-kernel - Re: [PATCH 6/7] drm/tilcdc: clean up ifdef hacks around iowrite64

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Message-ID: <CAK8P3a2HhO_zCnsTzq7hmWSz5La5Thu19FWZpun16iMnyyNreQ@mail.gmail.com>
Date:   Tue, 27 Jun 2017 22:40:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Jyri Sarha <jsarha@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-ntb@...glegroups.com, linux-alpha@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-crypto@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Stephen Bates <sbates@...thlin.com>,
        Tomi Valkeinen <tomi.valkeinen@...com>,
        David Airlie <airlied@...ux.ie>
Subject: Re: [PATCH 6/7] drm/tilcdc: clean up ifdef hacks around iowrite64

On Mon, Jun 26, 2017 at 6:26 PM, Logan Gunthorpe <logang@...tatee.com> wrote:
> Hi Jyri,
>
> Thanks for the ack. However, I'm reworking this patch set to use the
> include/linux/io-64-nonatomic* headers which will explicitly devolve
> into two 32-bit transfers. It's not clear whether this is appropriate
> for the tilcdc driver as it was never setup to use 32-bit transfers
> (unlike the others I had patched).
>
> If you think it's ok, I can still patch this driver to use the
> non-atomic headers. Otherwise I can leave it out. Please let me know.

You'd have to first figure out whether this device is of the lo-hi
or the hi-lo variant, or doesn't allow the I/O to be split at all.

Note that we could theoretically define ARM to use strd/ldrd
for writeq/readq, but I would expect that to be wrong with many
other devices that can use the existing io-64-nonatomic headers.

The comment in set_scanout() suggests that we actually do rely
on the write64 to be atomic, so we probably don't want to change
this driver.

         Arnd