linux-kernel - RE: Xilinx SYSACE driver and 8-bit attachment

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Message-ID: <893df3b8-adf9-4d96-a970-11d2412117e8@VA3EHSMHS026.ehs.local>
Date:	Wed, 7 Oct 2009 16:48:20 -0700
From:	Stephen Neuendorffer <stephen.neuendorffer@...inx.com>
To:	"John Williams" <john.williams@...alogix.com>,
	<grant.likely@...retlab.ca>
CC:	"Linux Kernel list" <linux-kernel@...r.kernel.org>,
	"Michal Simek" <michal.simek@...alogix.com>,
	"John Linn" <linnj@...inx.com>,
	"David DeBonis" <ddeboni@...inx.com>,
	"devicetree-discuss" <devicetree-discuss@...ts.ozlabs.org>
Subject: RE: Xilinx SYSACE driver and 8-bit attachment



> -----Original Message-----
> From: John Williams [mailto:john.williams@...alogix.com]
> Sent: Wednesday, October 07, 2009 4:45 PM
> To: grant.likely@...retlab.ca
> Cc: Linux Kernel list; Michal Simek; John Linn; David DeBonis; Stephen Neuendorffer; devicetree-
> discuss
> Subject: Re: Xilinx SYSACE driver and 8-bit attachment
> 
> On Wed, Oct 7, 2009 at 11:34 PM, Grant Likely <grant.likely@...retlab.ca> wrote:
> > [Added devicetree-discuss to cc: list]
> >
> > On Wed, Oct 7, 2009 at 1:13 AM, John Williams
> > <john.williams@...alogix.com> wrote:
> >> Hi Grant (it's your driver :) and David D ( and your DTS generator :)
> > [...]
> >> if (of_find_property(op->node, "8-bit", NULL))
> > [...]
> >> doesn't match the properties generated by Xilinx's device tree generator:
> > [...]
> >>                         xlnx,mem-width = <0x8>;
> >>
> >> So, the question is, which should change?
> >
> > Well, obviously its a device tree generator bug, the driver code is
> > perfect.  :-P
> 
> How did I know you'd say that? :)
> 
> > In all seriousness though, the '8-bit' property has been present for a
> > while now and needs to be retained to not break existing users.  It
> > would be okay for the driver to be modified to *also* check for the
> > xlnx,mem-width property, but the better solution is to modify the
> > device tree generator.  Plus, the '8-bit' property is the documented
> > binding in Documentation/powerpc/dts-bindings/xilinx.txt
> 
> I tend to agree - but I also tend think that putting funky, arbitrary
> output strings in the DTS generator is also a bad idea.
> 
> In this case, my vote is leaning towards adding a test for
> "xlnx,mem-width = <..>" to the driver, in addition to the existing
> "8-bit" tag to keep back compatability.
> 
> How would you feel about that?
> 
> In future, for new Xilinx drivers I think the default binding should
> be inspired by what comes from the DTS generator automatically, with
> out any special per-device hacks.  This has a natural tendency towards
> minimum pain I think.

I second this wholeheartedly.
Note that by definition, what comes out of the generator 'accurately and exactly describes the hardware', to the extent that the hardware is represented completely by its parameters.

Steve


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