Re: [PATCH 1/2] dma: add Qualcomm Technologies HIDMA management driver

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Al Stone <al.stone@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Sinan Kaya <okaya@codeaurora.org>,
	dmaengine <dmaengine@vger.kernel.org>,
	timur@codeaurora.org, cov@codeaurora.org, jcm@redhat.com,
	Rob Herring <robh+dt@kernel.org>, Pawel Moll <pawel.moll@arm.com>,
	Ian Campbell <ijc+devicetree@hellion.org.uk>,
	Kumar Gala <galak@codeaurora.org>,
	Vinod Koul <vinod.koul@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	devicetree <devicetree@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	leif.lindholm@linaro.org
Subject: Re: [PATCH 1/2] dma: add Qualcomm Technologies HIDMA management driver
Date: Fri, 30 Oct 2015 22:15:09 +0200	[thread overview]
Message-ID: <CAHp75VcQ6r2FGqsP2mtnv95etCGJmeQ=T7aKSB5H3cvwhdSbDw@mail.gmail.com> (raw)
In-Reply-To: <5633CE23.7060303@linaro.org>

On Fri, Oct 30, 2015 at 10:08 PM, Al Stone <al.stone@linaro.org> wrote:
> On 10/30/2015 01:01 PM, Mark Rutland wrote:
>> On Fri, Oct 30, 2015 at 02:48:06PM -0400, Sinan Kaya wrote:

>> The CSRT is listed under "Want", not "Never" or "Don't Care", so Linaro
>> have certainly not said that CSRT will not be supported. If anything,
>> they have stated that the table should be supported.
>
> "Want" means interesting, and probably useful, but no clear indication that
> anyone actually needs it.  At one point, we thought we might use the CSRT
> for describing DMA, but it turns out we have not needed to.

Then you are going to have either 1 or 0 DMAC for slave devices, right?

The CSRT, unfortunately, the only way how to enumerate DMAC to be used
by slave devices.
You may look into drivers/dma/acpi-dma.c for usage in Linux.

Yes, I know about _DSD, but I don't think it will provide soon any
other official mechanisms to what we have now. Better to ask Rafael
and Mika.

> However, let's make sure we're saying the same thing: the CSRT table is
> properly defined in the kernel include/acpi/actbl2.h file so one can read
> such a table and use it if they so choose.  Nothing that we have done at
> Linaro in the arm64 part of the kernel relies on any of the content from
> the CSRT, nor does it preclude someone relying on that content.  So, the
> CSRT is defined, and is usable, but is just not being used -- by Linaro --
> at present.

This sounds clear.

> If that needs to change, let me know; no one has asked us to use the CSRT
> for a DMA engine, and we have not been provided any hardware that requires
> it.

See above.

-- 
With Best Regards,
Andy Shevchenko