From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Dooks Subject: Re: [RFC] [PATCH] Device Tree on ARM platform Date: Thu, 28 May 2009 15:22:08 +0100 Message-ID: <20090528142208.GB16199@trinity.fluff.org> References: <20090527150527.GK6805@pengutronix.de> <87vdnm8sec.fsf@macbook.be.48ers.dk> <4A1D6901.2090508@freescale.com> <20090527175609.GB31861@flint.arm.linux.org.uk> <4A1D8FBA.6040802@freescale.com> <9e4733910905271213k7f4b93e7i7e6f2af24d85f@mail.gmail.com> <20090527192116.GA31110@n2100.arm.linux.org.uk> <20090527193927.GD30039@game.jcrosoft.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Grant Likely Cc: Jean-Christophe PLAGNIOL-VILLARD , Russell King - ARM Linux , devicetree-discuss , linux-kernel@vger.kernel.org, Timur Tabi , Jon Smirl , Scott Wood , Janboe Ye , linux-arm-kernel@lists.arm.linux.org.uk List-Id: devicetree@vger.kernel.org On Wed, May 27, 2009 at 02:22:50PM -0600, Grant Likely wrote: > On Wed, May 27, 2009 at 1:39 PM, Jean-Christophe PLAGNIOL-VILLARD > wrote: > > On 20:21 Wed 27 May =A0 =A0 , Russell King - ARM Linux wrote: > >> On Wed, May 27, 2009 at 03:13:55PM -0400, Jon Smirl wrote: > >> > On Wed, May 27, 2009 at 3:08 PM, Scott Wood wrote: > >> > > I'm not talking about platform specific code, I'm talking abou= t code to > >> > > retrieve information about a device from the device tree. =A0T= here would not > >> > > be separate instances of this for "platforms X, Y and Z", just= one > >> > > of_platform binding in each driver. =A0It's no different than = having a > >> > > platform bus binding, except in the data structures used. > >> > > > >> > > But to restate, having external glue to create platform device= s from the > >> > > device tree is fine if that's what you want to do. =A0We used = to do that, but > >> > > it was a pain compared to keeping everything in one place. =A0= Your experience > >> > > may differ. > >> > > >> > Could 'struct platform_device' and 'struct of_platform_device" b= e > >> > unified into a single structure? It's personal preference whethe= r the > >> > internal representation of the hardware is done via a device tre= e or > >> > snippets of platform code, but do we need to have to different d= evice > >> > types? > >> > >> That's a damned good question - platform devices have been around = since > >> the dawn of the device model, so the real question which needs to = be > >> asked is: what was the reason that of_platform_device created rath= er > >> than unifying it with the already provided platform_device ? > > I agree at 100% > > > > when you have to support the same driver for non OF and OF platform= it's > > really a pain in the ass >=20 > There are two issues that keep the of_platform and platform busses > separate. They aren't show stoppers, but they reflect the current > state. >=20 > 1) Source of data: a platform_device carries a pdata structure with i= t > to describe the hardware. An of_device carries a device_node pointer= =2E > Before dropping of_platform bus, a mechanism needs to be in place to > add hooks for translating the device tree data into a pdata structure > for each platform device. >=20 > 2) Driver binding mechanism: device tree nodes usually have a > "compatible" property which is a list of strings. The first string > describes exactly what the device is (ie. "atmel,24c08") and an > optional list of other devices which it is register interface > backwards compatible with. The intent is that newer devices can clai= m > compatibility with older ones so that existing device drivers will > work without needing to be told the new device name. However, it > leaves the option when a device errata or something similar raises > it's ugly head, a driver can still get information about the exact > device name and apply the appropriate workarounds. Driver probing > should walk the list and give preference to higher priority compatibl= e > values. of_platform bus does this, but I cannot think of a clean way > to do the same thing with the platform bus. >=20 > One option that has been suggested (more than once) is to make the > adapter code an of_platform_driver which translates the device tree > data and then registers the appropriate platform_devices. This neatl= y > solves the problem, but I don't like the overhead involved in > registering 2 struct devices with the kernel for every device node in > the device tree. Surely the code could simply run at init time, throwing away the data and code it doesn't need once it is done? --=20 Ben Q: What's a light-year? A: One-third less calories than a regular year.