linux-kernel - Re: [RFC PATCH 00/16] OMAP: GPMC: Restructure OMAP GPMC driver (NAND) : DT binding change proposal

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Message-ID: <20140522144600.GA1785@arch.cereza>
Date:	Thu, 22 May 2014 11:46:00 -0300
From:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
To:	Roger Quadros <rogerq@...com>, Tony Lindgren <tony@...mide.com>,
	Javier Martinez Canillas <javier@...hile0.org>
Cc:	Brian Norris <computersforpeace@...il.com>,
	"Gupta, Pekon" <pekon@...com>,
	Robert Nelson <robertcnelson@...il.com>,
	Jingoo Han <jg1.han@...sung.com>, dwmw2@...radead.org,
	nsekhar@...com,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	linux-mtd@...ts.infradead.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robh+dt@...nel.org>
Subject: Re: [RFC PATCH 00/16] OMAP: GPMC: Restructure OMAP GPMC driver
 (NAND) : DT binding change proposal

On 22 May 01:51 PM, Javier Martinez Canillas wrote:
> On Thu, May 22, 2014 at 10:12 AM, Roger Quadros <rogerq@...com> wrote:
> >> On 21 May 02:20 PM, Roger Quadros wrote:
> >>>
> >>> For DT boot:
> >>> - The GPMC controller node should have a chip select (CS) node for each used
> >>>   chip select. The CS node must have a child device node for each device
> >>>   attached to that chip select. Properties for that child are GPMC agnostic.
> >>>
> >>>   i.e.
> >>>      gpmc {
> >>>              cs0 {
> >>>                      nand0 {
> >>>                      }
> >>>              };
> >>>
> >>>              cs1 {
> >>>                      nor0 {
> >>>                      }
> >>>              }
> >>>              ...
> >>>      };
> >>>
> >>
> >> While I agree that the GPMC driver is a bit messy, I'm not sure it's possible
> >> to go through such a complete devicetree binding re-design (breaking backwards
> >> compatibility) now that the binding is already in production.
> >
> > Why not? especially if the existing bindings are poorly dones. Is anyone using these
> > bindings burning the DT into ROM and can't change it when they update the kernel?
> >
> 
> While I do agree that your DT bindings are much better than the
> current ones, there is a policy that DT bindings are an external API
> and once are released with a kernel are set in stone and can't be
> changed.
> 

Exactly. The DT binding is considered an ABI. Thus, invariant across kernel
versions. Users can't be coherced into a DTB update after a kernel update.

That said, I don't really care if you break compatilibity in this case.
Rather, I'm suggesting that you make sure this change is going to be accepted
upstream, before doing any more work. The DT maintainers are reluctant to do
so.

On the other side, I guess you will also break bisectability while breaking
backward compatibility. Doesn't sound very nice.
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
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