Date:	Fri, 21 Mar 2014 08:14:44 -0700
From:	Byungho An <bh74.an@...sung.com>
To:	'Mark Rutland' <mark.rutland@....com>
Cc:	netdev@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
	devicetree@...r.kernel.org, davem@...emloft.net,
	ilho215.lee@...sung.com, siva.kallam@...sung.com,
	vipul.pandya@...sung.com, ks.giri@...sung.com
Subject: RE: [PATCH V4 1/8] sxgbe: Add device-tree binding support document


Mark Rutland <mark.rutland@....com> wrote :
> On Wed, Mar 19, 2014 at 10:32:48PM +0000, Byungho An wrote:
> > Mark Rutland <mark.rutland@....com> :
> > > On Tue, Mar 18, 2014 at 04:27:46PM +0000, Byungho An wrote:
> > > > Mark Rutland <mark.rutland@....com> :
> > > > > Hi,
> > > > >
> > > > > As a general note it's helpful for devicetree to be Cc'd on the
> > > > > entire
> > > > series
> > > > > (though the binding document should be a separate patch) as it
> > > > > provides
> > > > useful
> > > > > context for reviewing the binding.
> > > > OK.
> > > >
> > > > >
> > > > > On Tue, Mar 18, 2014 at 06:47:13AM +0000, Byungho An wrote:
> > > > > > From: Siva Reddy <siva.kallam@...sung.com>
> > > > > >
> > > > > > This patch adds binding document for SXGBE ethernet driver via
> > > > device-tree.
> > > > > >
> > > > > > Signed-off-by: Siva Reddy Kallam <siva.kallam@...sung.com>
> > > > > > Signed-off-by: Byungho An <bh74.an@...sung.com>
> > > > > > ---
> > > > > >  .../devicetree/bindings/net/samsung-sxgbe.txt      |   53
> > > > > > ++++++++++++++++++++
> > > > > >  1 file changed, 53 insertions(+)  create mode 100644
> > > > > > Documentation/devicetree/bindings/net/samsung-sxgbe.txt
> > > > > >
> > > > > > diff --git
> > > > > > a/Documentation/devicetree/bindings/net/samsung-sxgbe.txt
> > > > > > b/Documentation/devicetree/bindings/net/samsung-sxgbe.txt
> > > > > > new file mode 100644
> > > > > > index 0000000..ca27947
> > > > > > --- /dev/null
> > > > > > +++ b/Documentation/devicetree/bindings/net/samsung-sxgbe.txt
> > > > > > @@ -0,0 +1,53 @@
> > > > > > +* Samsung 10G Ethernet driver (SXGBE)
> > > > > > +
> > > > > > +Required properties:
> > > > > > +- compatible: Should be "samsung,sxgbe-v2.0a"
> > > > > > +- reg: Address and length of the register set for the device
> > > > > > +- interrupt-parent: Should be the phandle for the interrupt
> > > > > > +controller
> > > > > > +  that services interrupts for this device
> > > > > > +- interrupts: Should contain the SXGBE interrupts
> > > > > > +  These interrupts are ordered by fixed and follows variable
> > > > > > +  trasmit DMA interrupts, receive DMA interrupts and lpi
interrupt.
> > > > > > +  index 0 - this is fixed common interrupt of SXGBE and it is
> > > > > > +always
> > > > > > +  available.
> > > > > > +  index 1 to 25 - 8 variable trasmit interrupts, variable 16
> > > > > > +receive
> > > > > > interrupts
> > > > > > +  and 1 optional lpi interrupt.
> > > > > > +- phy-mode: String, operation mode of the PHY interface.
> > > > > > +  Supported values are: "xaui", "gmii".
> > > > > > +- samsung,pbl: Integer, Programmable Burst Length.
> > > > > > +  Supported values are 1, 2, 4, 8, 16, or 32.
> > > > >
> > > > > There's no need to abbreviate to "pbl".
> > > > >
> > > > > Is this a property of the hardware, or configuration that the
> > > > > kernel will
> > > > program
> > > > > in? If the latter, why can the kernel not choose?
> > > > Yes, this is hardware property
> > >
> > > Ok.
> > >
> > > >
> > > > >
> > > > > > +- samsung,fixed-burst: Boolean, Program the DMA to use the
> > > > > > +fixed burst mode
> > > > > > +- samsung,burst-map: Integer, Program the possible bursts
> > > > > > +supported by sxgbe
> > > > > > +  This is an interger and represents allowable DMA bursts
> > > > > > +when fixed
> > > > burst.
> > > > > > +  Allowable range is 0x00-0x3F. This field is valid only when
> > > > > > +fixed burst is
> > > > > > +  enabled, otherwise ignored.
> > > > >
> > > > > If that's the case, why not have just this property and have it
> > > > > imply the
> > > > use of
> > > > > fixed burst mode?
> > > > OK. It will be implemented in next patch set.
> > > >
> > > > >
> > > > > When is it necessary to use fixed burst mode?
> > > > This is the configurable mode of DMA an used internally by
> > > > hardware to fetch data from platform bus
> > >
> > > Sure, but that doesn't describe when it is necessary. Is this the
> > > way the
> > DMA
> > > was configured at integration time, or the way the kernel should
> > > configure
> > it?
> > It is needed when fixed length of burst is needed.
> > if it is not configured, the length of burst will be variable(not fixed).
> > Anyway, I'll add description more for it.
> 
> And when is it necessary to have a fixed length of burst?
It's up to situation. If most of data size have similar in size,  fixed burst
is better.

> 
> Is this a property of the system, or the conenction of the system to
another?
This is a choice given by SXGBE to configure it's burst mode. It can work in
Fixed and Undefined burst mode. 
SXGBE can't select the burst mode on it's own. so, to configure the burst , we
need configurable parameter from DT. 
if DT doesn't provide fixed burst, it means SXGBE driver will configure as
Undefined burst

Do you think it should be moved to optional properties?

> 
> >
> > >
> > > If the latter, is it absolutely necessary for correctness to use
> > > fixed-burst
> > mode?
> > > Or is it just always sensible to use it if available?
> > It is not absolutely necessary, as I mentioned above.
> 
> The description above does not make this clear.
> 
> >
> > >
> > > What does the driver do if fixed burst mode is not available? Would
> > > this
> > work in
> > > the presence of fixed-burt mode?
> > Fixed burst mode is always available so driver doesn't need to care about
it.
> >
> > >
> > > I'm not arguing to remove these properties. I'd just like to
> > > understand if
> > all
> > > you're describing is the presence of a feature or that the use of
> > > the
> > feature is
> > > absolutely necessary for correctness.
> > OK
> >
> > >
> > > I'm perfectly happy for Linux to always decide to use these features
> > > if
> > available.
> > >
> > > >
> > > > >
> > > > > > +- samsung,adv-addr-mode: Boolean, Program the DMA to use
> > > > > > +Enhanced address
> > > > > > mode.
> > > > >
> > > > > When would this be selected, and why can the kernel not choose?
> > > > Kernel doesn't have the provision to find out a way to select this.
> > > > So, need to pass from DT
> > >
> > > Likewise, it is always absolutely necessary, or just always sensible
> > > to use enhanced address mode if present?
> > Not always necessary. When extended address mode is needed, it can be
> > set in the DT.
> 
> When is this needed?
It is always confusing between when and why.
It is for extended dma addressing  for future. 
when extended dma addressing is needed it can be selected.
Anyway it will be removed this series because it is not mandatory.
Should I explain about extended dma addressing?

> 
> Cheers,
> Mark.
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