netdev - Re: [PATCH net-next v4 1/2] dts: Add a binding for Synopsys emac max-frame-size

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Date:	Thu, 16 Jan 2014 19:45:23 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Rob Herring <robherring2@...il.com>
CC:	Vince Bridgers <vbridgers2013@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	<netdev@...r.kernel.org>,
	Giuseppe CAVALLARO <peppe.cavallaro@...com>,
	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>,
	"Dinh Nguyen" <dinguyen@...era.com>,
	Rayagond Kokatanur <rayagond@...avyalabs.com>
Subject: Re: [PATCH net-next v4 1/2] dts: Add a binding for Synopsys emac
 max-frame-size

On Thu, 2014-01-16 at 13:29 -0600, Rob Herring wrote:
> On Thu, Jan 16, 2014 at 12:48 PM, Vince Bridgers
> <vbridgers2013@...il.com> wrote:
> > On Thu, Jan 16, 2014 at 11:50 AM, Ben Hutchings
> > <bhutchings@...arflare.com> wrote:
> >> On Thu, 2014-01-16 at 08:05 -0600, Vince Bridgers wrote:
> >>> This change adds a parameter for the Synopsys 10/100/1000
> >>> stmmac Ethernet driver to configure the maximum frame
> >>> size supported by the EMAC driver. Synopsys allows the FIFO
> >>> sizes to be configured when the cores are built for a particular
> >>> device, but do not provide a way for the driver to read
> >>> information from the device about the maximum MTU size
> >>> supported as limited by the device's FIFO size.
> >>>
> >>> Signed-off-by: Vince Bridgers <vbridgers2013@...il.com>
> >>> ---
> >>> V4: add comments to explain use of max-frame-size with respect
> >>>     to inconsistent definition and use in the ePAPR v1.1 spec
> >>
> >> Well, ePAPR does not seem to be consistent with itself. :-)
> >>
> >>> V3: change snps,max-frame-size to max-frame-size
> >>> V2: change snps,max-mtu to snps,max-frame-size
> >>> ---
> >>>  Documentation/devicetree/bindings/net/stmmac.txt |   13 ++++++++++++-
> >>>  1 file changed, 12 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/net/stmmac.txt b/Documentation/devicetree/bindings/net/stmmac.txt
> >>> index eba0e5e..d553be2 100644
> >>> --- a/Documentation/devicetree/bindings/net/stmmac.txt
> >>> +++ b/Documentation/devicetree/bindings/net/stmmac.txt
> >>> @@ -29,7 +29,17 @@ Required properties:
> >>>                               ignored if force_thresh_dma_mode is set.
> >>>
> >>>  Optional properties:
> >>> -- mac-address: 6 bytes, mac address
> >>> +- mac-address:               6 bytes, mac address
> >>> +- max-frame-size:    Maximum frame size permitted. This parameter is useful
> >>> +                     since different implementations of the Synopsys MAC may
> >>> +                     have different FIFO sizes depending on the selections
> >>> +                     made in Synopsys Core Consultant. Note that the usage
> >>> +                     is inconsistent with the definition in the ePAPR v1.1
> >>> +                     specification, as it defines max-frame-size inclusive
> >>> +                     of the MAC DA, SA, Ethertype and CRC while usage is
> >>> +                     consistent with the IEEE definition of MAC Client
> >>> +                     Data, which is sans the MAC DA, SA, Ethertype and
> >>> +                     CRC.
> >> [...]
> >>
> >> While this is very precise, I fear that it is now so verbose that it
> >> actually becomes confusing.  Can this not be condensed to 'the maximum
> >> MTU and MRU, rather than the maximum Ethernet frame size'?
> >>
> >> Ben.
> >
> > Sure, I'll cut this down and resubmit as V5. How about
> >
> > "The Maximum Transfer Unit (IEEE defined MTU), rather than the maximum
> > frame size."
> 
> If you are not following the ePAPR definition, then call the property
> something else rather than relying on the documentation to explain the
> different meaning. Why can't the driver adjust the size from the ePAPR
> definition? Otherwise, I would go back to the v2 name of
> "snps,max-mtu".

ePAPR *doesn't have* a clear definition:
http://article.gmane.org/gmane.linux.network/300098

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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