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

From: Mark Rutland <mark.rutland@arm.com>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: 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>
Subject: Re: [PATCH 1/2] dma: add Qualcomm Technologies HIDMA management driver
Date: Fri, 30 Oct 2015 18:25:37 +0000	[thread overview]
Message-ID: <20151030182537.GB30791@leverpostej> (raw)
In-Reply-To: <CAHp75Ve_7NeyJD==EpgzKes9n9wb5oTwtNt7FvZHUSUxqZ4+=Q@mail.gmail.com>

On Fri, Oct 30, 2015 at 08:18:49PM +0200, Andy Shevchenko wrote:
> On Fri, Oct 30, 2015 at 8:08 PM, Sinan Kaya <okaya@codeaurora.org> wrote:
> 
> >> The ACPI tables DSDT and CSRT (more info here:
> >> http://www.acpi.info/links.htm) defines properties.
> >>
> >> DSDT:
> >>   per DMAC: the resources
> >>   per client: FixedDMA descriptor that contains channel / request line
> >> pair.
> >>
> >> CSRT:
> >>   necessary table to map which DMAC provides which request line, thus
> >> request line numbering are global on platform.
> >>
> >> When DMAC driver is probed in the running system it should call as
> >> well registration function from acpi-dma.c.
> >>
> >> All clients when use new DMA slave API gets channel automatically
> >> based on their FixedDMA property.
> >>
> >> So, above is how it should be done. Didn't actually checked what this
> >> driver does.
> >>
> > I was going to reply to all the questions in one pass but let me handle
> > piece by piece.
> >
> > Here are some facts.
> > - This hardware supports memcpy and memset only.
> > - Memset feature was removed from the kernel sometime around 3.14. So no
> > memset support in this driver either.
> > - The hardware does not support DMA slave support
> > - The goal is to provide an interface to DMA engine framework for memcpy
> > optimization so that the rest of the kernel drivers and applications make
> > use of the hardware.
> >
> 
> > CSRT is an Intel specific ACPI table for slave devices.
> 
> Wrong.
> It was designed by Microsoft to support multiple controllers, in
> particular DMACs.
> Have you read that document I posted link to?
> 
> > It was decided by
> > Linaro that CSRT will not be supported for ARM64.
> 
> Interesting, ARM64 platforms are not going to have more than one DMAC
> per system?

I cannot imagine that being true, and I don't see why Linaro would
decide such a thing.

It does appear that it's not relevant to this device and driver, given
the lack of clients, unless I've misunderstood?

Thanks,
Mark.