linux-kernel - Re: [PATCH v6 2/2] media: V3s: Add support for Allwinner CSI.

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Message-ID: <20180130095916.GA23047@ulmo>
Date:   Tue, 30 Jan 2018 10:59:16 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Yong Deng <yong.deng@...ewell.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Chen-Yu Tsai <wens@...e.org>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hans Verkuil <hans.verkuil@...co.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Hugues Fruchet <hugues.fruchet@...com>,
        Yannick Fertre <yannick.fertre@...com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Ramesh Shanmugasundaram <ramesh.shanmugasundaram@...renesas.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Rick Chang <rick.chang@...iatek.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>, megous@...ous.com,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: Re: [PATCH v6 2/2] media: V3s: Add support for Allwinner CSI.

On Tue, Jan 30, 2018 at 10:24:48AM +0100, Arnd Bergmann wrote:
> On Tue, Jan 30, 2018 at 8:54 AM, Maxime Ripard
> <maxime.ripard@...e-electrons.com> wrote:
> > On Mon, Jan 29, 2018 at 03:34:02PM +0100, Arnd Bergmann wrote:
> >> On Mon, Jan 29, 2018 at 10:25 AM, Linus Walleij
> >> <linus.walleij@...aro.org> wrote:
> >> > On Mon, Jan 29, 2018 at 9:25 AM, Maxime Ripard
> >> > <maxime.ripard@...e-electrons.com> wrote:
> >> >> On Sat, Jan 27, 2018 at 05:14:26PM +0100, Linus Walleij wrote:
> 
> >>
> >> At one point we had discussed adding a 'dma-masters' property that
> >> lists all the buses on which a device can be a dma master, and
> >> the respective properties of those masters (iommu, coherency,
> >> offset, ...).
> >>
> >> IIRC at the time we decided that we could live without that complexity,
> >> but perhaps we cannot.
> >
> > Are you talking about this ?
> > https://elixir.free-electrons.com/linux/latest/source/Documentation/devicetree/bindings/dma/dma.txt#L41
> >
> > It doesn't seem to be related to that issue to me. And in our
> > particular cases, all the devices are DMA masters, the RAM is just
> > mapped to another address.
> 
> No, that's not the one I was thinking of. The idea at the time was much
> more generic, and not limited to dma engines. I don't recall the details,
> but I think that Thierry was either involved or made the proposal at the
> time.

Yeah, I vaguely remember discussing something like this before. A quick
search through my inbox yielded these two threads, mostly related to
IOMMU but I think there were some mentions about dma-ranges and so on as
well. I'll have to dig deeper into those threads to refresh my memories,
but I won't get around to it until later today.

If someone wants to read up on this in the meantime, here are the links:

	https://lkml.org/lkml/2014/4/27/346
	http://lists.infradead.org/pipermail/linux-arm-kernel/2014-May/257200.html

From a quick glance the issue of dma-ranges was something that we hand-
waved at the time.

Thierry

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