Re: [PATCH v7 2/2] memory: atmel-ebi: add DT bindings documentation

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Tue, 10 May 2016 12:07:42 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> On Tue, May 10, 2016 at 10:04:48AM +0200, Boris Brezillon wrote:
> > On Wed, 4 May 2016 15:35:47 +0200
> > Boris Brezillon <boris.brezillon@free-electrons.com> wrote:
> >   
> > > On Wed, 4 May 2016 08:06:10 -0500
> > > Rob Herring <robh@kernel.org> wrote:
> > >   
> > > > On Wed, May 4, 2016 at 4:38 AM, Boris Brezillon
> > > > <boris.brezillon@free-electrons.com> wrote:    
> > > > > Hi Rob,
> > > > >
> > > > > On Tue, 3 May 2016 14:11:04 -0500
> > > > > Rob Herring <robh@kernel.org> wrote:
> > > > >      
> > > > >> On Tue, May 3, 2016 at 11:51 AM, Boris Brezillon
> > > > >> <boris.brezillon@free-electrons.com> wrote:      
> > > > >> > Hi Rob,
> > > > >> >
> > > > >> > On Tue, 3 May 2016 11:40:19 -0500
> > > > >> > Rob Herring <robh@kernel.org> wrote:
> > > > >> >      
> > > > >> >> On Thu, Apr 28, 2016 at 02:03:27PM +0200, Boris Brezillon wrote:      
> > > > >> >> > The EBI (External Bus Interface) is used to access external peripherals
> > > > >> >> > (NOR, SRAM, NAND, and other specific devices like ethernet controllers).
> > > > >> >> > Each device is assigned a CS line and an address range and can have its
> > > > >> >> > own configuration (timings, access mode, bus width, ...).
> > > > >> >> > This driver provides a generic DT binding to configure a device according
> > > > >> >> > to its requirements.
> > > > >> >> > For specific device controllers (like the NAND one) the SMC timings
> > > > >> >> > should be configured by the controller driver through the matrix and smc
> > > > >> >> > syscon regmaps.      
> > > > 
> > > > [...]
> > > >     
> > > > >> >> > +EBI bus configuration associated with specific chip-select will be defined in
> > > > >> >> > +the configs subnode. This configs node will in turn contain several subnodes
> > > > >> >> > +named config-<cs-id>, each of them containing the following properties.      
> > > > >> >>
> > > > >> >> This is a bit unusual. Why not just part of the child device nodes?      
> > > > >> >
> > > > >> > Oh, come on! I reworked the binding because Mark complained about the
> > > > >> > previous binding which was doing exactly what you're suggesting. Can
> > > > >> > you please be consistent in your reviews...      
> > > > >>
> > > > >> No, Mark and I both have our opinions. Which part of this patch
> > > > >> explains the history?      
> > > > >
> > > > > Hm, it's in patch 1/2 (just dropped the cover letter, which might not
> > > > > be such a good idea).
> > > > >      
> > > > >> If the revision history is not in the patch, I'm
> > > > >> not looking at it.
> > > > >>
> > > > >> My issue with it this way is that it has invented yet another way to
> > > > >> describe timings. I would like to be consistent across external bus
> > > > >> descriptions, but we're not very consistent to begin with though. The
> > > > >> most common seems to be the way you first did it. But I agree that it
> > > > >> is kind of screwy to have an intermediate node unless the controller
> > > > >> itself has sub-blocks within it and is not the established way to
> > > > >> describe a bus with chip selects. I would either put the properties
> > > > >> directly in the child nodes (e.g. flash@0,0) or put your config nodes
> > > > >> in the device node. I'd call it timings instead of config, but that's
> > > > >> just bikeshedding.      
> > > > >
> > > > > Well, it's not only describing timings (see atmel,bus-width,
> > > > > atmel,byte-access-type, ...), but I'm fine with either names :).
> > > > >      
> > > > >>
> > > > >> memory-controller@1000 {
> > > > >>   ...
> > > > >>   flash@0,0 {
> > > > >>     timings {
> > > > >>       ...
> > > > >>     };
> > > > >>   };
> > > > >> };      
> > > > >
> > > > > Okay. Mark, what do you think of this approach?
> > > > >
> > > > > Note that one of my previous version was defining timings directly in
> > > > > the EBI device node, and Arnd noted that doing so may cause problems
> > > > > if one of the EBI property (or the config/timing node name) conflict
> > > > > with the sub-device binding, which is why I decided to put the EBI
> > > > > config definitions in a separate subnode.      
> > > > 
> > > > You have vendor prefixes on all the properties so I don't think a
> > > > collision is really a problem. It's also an established pattern in
> > > > i.MX WEIM and OMAP GPMC (which are hiding in bindings/bus/) and I
> > > > prefer consistency.    
> > > 
> > > So let's summarize that.
> > > 
> > > memory-controller@1000 {
> > > 	...
> > > 	flash@0,0 {
> > > 		atmel,<ebi-prop-name> = <value>;
> > > 		...
> > > 		<flash-device-prop> = <value>;
> > > 	};
> > > };
> > > 
> > > Would everyone agree on this representation?
> > > 
> > > With this approach, it's a bit more complicated to detect the case
> > > where we want to keep bootloader/firmware config, but it should be
> > > doable (it's much more easier to test for the presence of a
> > > config/timing node than verifying that either all or none of the
> > > mandatory properties are here).
> > > 
> > > Still remains the problem mentioned by Jean-Jacques: what if the
> > > sub-device takes 2 CS lines. Should we apply the same setting to those
> > > slots?
> > >   
> > 
> > Rob, Mark, Arnd, can you take a decision regarding this binding? This
> > driver is floating around for quite some time, and we were asked to
> > rework the binding several times (in time in an opposite direction).
> > 
> > For the record, here is the thread I mentioned earlier [1]. In his
> > answer, Arnd suggests to put timing and bus config description
> > outside of the sub-device node. Mark recently complained about this
> > representation, which led me to the configs/config-X appraoch, and now
> > Rob suggests to go back to the first proposal.
> > 
> > I'm fine doing that, but can you please all confirm that you agree on
> > this binding?  
> 
> Sorry for the delay in getting round to this, and sorry that this
> appears to be going in circles.
> 
> Please go with Rob's suggestion.

Okay. This changes a bit the constraints defined in the binding doc
(no default values for undefined properties: we just keep the
bootloader/firmware config), but otherwise should be easy to implement.

> 
> I'm not sure about the case where a device takes 2 CS lines. I would
> assume that in practice that a sub-device covered my multiple CS lines
> expects the same timings for all its MMIO space, and so having that
> uniform makes sense. Do we have a counter-example?

Nope, I don't. JJH had one (interfacing with an FPGA), maybe he can
detail this use case.


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com