linux-kernel - Re: [RFC 04/10] memory: Add Tegra124 memory controller support

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Message-ID: <53ADE328.1080005@wwwdotorg.org>
Date:	Fri, 27 Jun 2014 15:33:28 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Thierry Reding <thierry.reding@...il.com>,
	Hiroshi DOyu <hdoyu@...dia.com>
CC:	Rob Herring <robh+dt@...nel.org>, Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	Will Deacon <will.deacon@....com>,
	Joerg Roedel <joro@...tes.org>,
	Cho KyongHo <pullip.cho@...sung.com>,
	Grant Grundler <grundler@...omium.org>,
	Dave Martin <Dave.Martin@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	Olav Haugan <ohaugan@...eaurora.org>,
	Paul Walmsley <pwalmsley@...dia.com>,
	Rhyland Klein <rklein@...dia.com>,
	Allen Martin <AMartin@...dia.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 04/10] memory: Add Tegra124 memory controller support

On 06/27/2014 05:08 AM, Thierry Reding wrote:
> On Fri, Jun 27, 2014 at 12:46:38PM +0300, Hiroshi DOyu wrote:
>>
>> Thierry Reding <thierry.reding@...il.com> writes:
>>
>>> From: Thierry Reding <treding@...dia.com>
>>>
>>> The memory controller on NVIDIA Tegra124 exposes various knobs that can
>>> be used to tune the behaviour of the clients attached to it.
>>>
>>> Currently this driver sets up the latency allowance registers to the HW
>>> defaults. Eventually an API should be exported by this driver (via a
>>> custom API or a generic subsystem) to allow clients to register latency
>>> requirements.
>>>
>>> This driver also registers an IOMMU (SMMU) that's implemented by the
>>> memory controller.
>>>
>>> Signed-off-by: Thierry Reding <treding@...dia.com>
>>> ---
>>>  drivers/memory/Kconfig                   |    9 +
>>>  drivers/memory/Makefile                  |    1 +
>>>  drivers/memory/tegra124-mc.c             | 1945 ++++++++++++++++++++++++++++++
>>>  include/dt-bindings/memory/tegra124-mc.h |   30 +
>>>  4 files changed, 1985 insertions(+)
>>>  create mode 100644 drivers/memory/tegra124-mc.c
>>>  create mode 100644 include/dt-bindings/memory/tegra124-mc.h
>>
>> I prefer reusing the existing SMMU and having MC and SMMU separated
>> since most of SMMU code are not different from functionality POV, and
>> new MC features are quite independent of SMMU.
>>
>> If it's really convenient to combine MC and SMMU into one driver, we
>> could move "drivers/iomm/tegra-smmu.c" here first, and add MC features
>> on the top of it.
> 
> I'm not sure if we can do that, since the tegra-smmu driver is
> technically used by Tegra30 and Tegra114. We've never really made use of
> it, but there are device trees in mainline releases that contain the
> separate SMMU node.

The existing DT nodes do nothing more than instantiate the driver.
However, IIUC nothing actually uses the driver for any purpose, so if we
simply deleted those nodes or changed them incompatibly, there'd be no
functional difference. Perhaps this is stretching DT ABIness very
slightly, but I think it makes no practical difference.


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