Message-ID: <20130912171541.GH22013@e106331-lin.cambridge.arm.com>
Date:	Thu, 12 Sep 2013 18:15:41 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Hongbo Zhang <hongbo.zhang@...escale.com>
Cc:	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Pawel Moll <Pawel.Moll@....com>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"ian.campbell@...rix.com" <ian.campbell@...rix.com>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"djbw@...com" <djbw@...com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 2/3] DMA: Freescale: Add new 8-channel DMA engine
 device tree nodes

On Tue, Sep 03, 2013 at 10:01:50AM +0100, Hongbo Zhang wrote:
> On 09/02/2013 11:58 PM, Mark Rutland wrote:
> > Hi,
> >
> > On Fri, Aug 30, 2013 at 12:26:19PM +0100, hongbo.zhang@...escale.com wrote:
> >> From: Hongbo Zhang <hongbo.zhang@...escale.com>
> >>
> >> Freescale QorIQ T4 and B4 introduce new 8-channel DMA engines, this patch adds
> >> the device tree nodes for them.
> >>
> >> Signed-off-by: Hongbo Zhang <hongbo.zhang@...escale.com>
> >> ---
> >>   .../devicetree/bindings/powerpc/fsl/dma.txt        |   67 ++++++++++++++++
> >>   arch/powerpc/boot/dts/fsl/b4si-post.dtsi           |    4 +-
> >>   arch/powerpc/boot/dts/fsl/elo3-dma-0.dtsi          |   82 ++++++++++++++++++++
> >>   arch/powerpc/boot/dts/fsl/elo3-dma-1.dtsi          |   82 ++++++++++++++++++++
> >>   arch/powerpc/boot/dts/fsl/t4240si-post.dtsi        |    4 +-
> >>   5 files changed, 235 insertions(+), 4 deletions(-)
> >>   create mode 100644 arch/powerpc/boot/dts/fsl/elo3-dma-0.dtsi
> >>   create mode 100644 arch/powerpc/boot/dts/fsl/elo3-dma-1.dtsi
> >>
> >> diff --git a/Documentation/devicetree/bindings/powerpc/fsl/dma.txt b/Documentation/devicetree/bindings/powerpc/fsl/dma.txt
> >> index ddf17af..332ac77 100644
> >> --- a/Documentation/devicetree/bindings/powerpc/fsl/dma.txt
> >> +++ b/Documentation/devicetree/bindings/powerpc/fsl/dma.txt
> >> @@ -126,6 +126,73 @@ Example:
> >>                  };
> >>          };
> >>
> >> +** Freescale Elo3 DMA Controller
> >> +   This is EloPlus controller with 8 channels, used in Freescale Txxx and Bxxx
> > I was under the impression EloPlus was the previous revision. Should
> > that say Elo3, or is Elo3 considered to be an EloPlus implementation?
> In this patch 1/3 I revise the doc to make it clear we have Elo and
> EloPlus, and I'm adding another new Elo3. Yes the only difference
> between Elo3 and EloPlus is channel numbers(8 channels vs 4 channels),
> so we can call "Elo3 is an 8-channel EloPlus"

Ok.

> >> +   series chips, such as t1040, t4240, b4860.
> >> +
> >> +Required properties:
> >> +
> >> +- compatible        : must include "fsl,elo3-dma"
> >> +- reg               : <registers specifier for DMA general status reg>
> > The example has two reg entries. What both are should be specified. From
> > what you described last time, it sounds like each is a status register
> > for four channels.
> >
> > Presumably the first covers the channels at 0x0,0x80,0x100,0x180, and
> > the second covers the channels at 0x300,0x380,0x400,0x480? If the
> > registers have specific names in a datasheet, it would be worth
> > mentioning them.
> Yes, each is a status register for four channels, you got it -- this
> means my statement works.
> Is it necessary to specify all the register names?
> I can describe my two registers, but in other cases the reg entryies can
> cover tens even hundreds of registers, just a summary is OK I think.

I think there should at least be a description of which channels each
reg entry corresponds to. I see this hasn't been done so far for the
older Elo DMAs, but they only had 4 channels max, and one status reg.

> > If the specification of the DMA controller allows for more channels, it
> > may be worth describing that case now.
> This DMA controller doesn't allows for more channels. (Even if it does,
> it should be another new controller)

Ok.

> >> +- ranges            : describes the mapping between the address space of the
> >> +                      DMA channels and the address space of the DMA controller
> > This looks odd as a required property, and I'm slightly confused. Is
> > this used to map the reg values of the DMA channels, or is it used when
> > mapping the DMA address space (for which dma-ranges exists in ePAPR and
> > other bindings).
> It is used to map the reg values of DMA channels.

Ok, I guess that makes sense.

> >> +
> >> +- DMA channel nodes:
> >> +        - compatible        : must include "fsl,eloplus-dma-channel"
> >> +        - reg               : <registers specifier for channel>
> > What does this represent? What are valid values?
> >
> > In the example below it looks like these are offsets of control
> > registers within the dma controller.
> Yes, they are offsets of control registers within dma controller, but
> the contents in these registers are for dma channels.
> Physically we have dma controller registers and dma channel registers,
> they are in one continuous physical address space, we divide all these
> registers into two controller/channel parts, according to contents in
> these registers, common status registers for all channels are called dma
> controller registers, otherwise channel specific registers are called
> dma channel registers.

I see, so this reg represents a channels channel specific registers
(which are distinct from the shared status registers). I was confused
initially as to what address space they were in, but that makes sense
with your description of ranges above.

> > If the reg property may have any value, how do they get mapped to bits
> > in the status register(s)?
> In fact, each channel has its own status register(and also other
> registers), the dma controller status register is just aggregation of
> all channel status register. (that seems duplicated somehow, maybe this
> is due to hardware compatibility with legacy one, and the device tree
> just describes the physical hardware without lie)

My question here was stupid, thanks for the explanation :)

> > May some channels be unusable for some reason, or will all eight
> > channels be wired on any given Elo3 DMA?
> Sorry, not get your point clearly, maybe you are clear now because of my
> previous explanations.

I assume that on any El03 DMA, there won't be a case where you can't
describe the channel at 0x80, for instance. It will always be present
(but it might not be wired up to anything any therefore be useful)?

This was related to my concerns about the status register description --
if the channels at 0x0,0x80,0x100,0x180 weren't wired, what would get
described in the dt? I guess that would never actually happen because
all 8 channels must always be present in the Elo3 IP block.

Thanks,
Mark.
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