Message-ID: <20150406150442.GA26319@beef>
Date:	Mon, 6 Apr 2015 11:04:42 -0400
From:	Matt Porter <mporter@...sulko.com>
To:	Rob Herring <robherring2@...il.com>
Cc:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v4 05/10] eeprom: Add bindings for simple eeprom framework

On Mon, Apr 06, 2015 at 09:11:05AM -0500, Rob Herring wrote:
> On Mon, Apr 6, 2015 at 8:32 AM, Matt Porter <mporter@...sulko.com> wrote:
> > On Mon, Mar 30, 2015 at 10:57:59PM +0100, Srinivas Kandagatla wrote:
> >> This patch adds bindings for simple eeprom framework which allows eeprom
> >> consumers to talk to eeprom providers to get access to eeprom cell data.
> >>
> >> Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
> >> [Maxime Ripard: intial version of eeprom framework]
> >> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
> >> ---
> >>  .../devicetree/bindings/eeprom/eeprom.txt          | 58 ++++++++++++++++++++++
> >>  1 file changed, 58 insertions(+)
> >>  create mode 100644 Documentation/devicetree/bindings/eeprom/eeprom.txt
> >>
> >> diff --git a/Documentation/devicetree/bindings/eeprom/eeprom.txt b/Documentation/devicetree/bindings/eeprom/eeprom.txt
> >> new file mode 100644
> >> index 0000000..fb71d46
> >> --- /dev/null
> >> +++ b/Documentation/devicetree/bindings/eeprom/eeprom.txt
> >> @@ -0,0 +1,58 @@
> >> += EEPROM Data Device Tree Bindings =
> >> +
> >> +This binding is intended to represent the location of hardware
> >> +configuration data stored in EEPROMs.
> >> +
> >> +On a significant proportion of boards, the manufacturer has stored
> >> +some data on an EEPROM-like device, for the OS to be able to retrieve
> >> +these information and act upon it. Obviously, the OS has to know
> >> +about where to retrieve these data from, and where they are stored on
> >> +the storage device.
> >
> > Since this binding (and the kernel framework supporting it) describes
> > non-volatile memory devices other than EEPROMs (e.g. EFuses) it should
> > be named more generically like "nvmem".
> >
> >> +
> >> +This document is here to document this.
> >> +
> >> += Data providers =
> >> +Contains bindings specific to provider drivers and data cells as children
> >> +to this node.
> >> +
> >> += Data cells =
> >> +These are the child nodes of the provider which contain data cell
> >> +information like offset and size in eeprom provider.
> >> +
> >> +Required properties:
> >> +reg: specifies the offset in byte within that storage device, and the length
> >> +     in bytes of the data we care about.
> >> +     There could be more then one offset-length pairs in this property.
> >> +
> >> +Optional properties:
> >> +As required by specific data parsers/interpreters.
> >
> > The generic binding could really use a "read-only" property here as this
> > is a common hardware attribute for many nvmem devices. A serial EEPROM
> > commonly has a write protect pin which may be hard-wired such that the
> > hardware description should reflect that. An EFuse is typically blown with
> > the required information at manufacturing time (for an end product case)
> > and would be marked with the "read-only" flag.
> >
> > Having this optional flag in the generic binding would allow the
> > framework to hint to consumers about the inability to write to the
> > provided region.
> 
> This could get fairly complex if you wanted to describe grouping of WP
> regions which could have different layout than the fields here. This
> may be better left as a device property listing addr & size pairs.
> However, there is the notion of s/w "read-only" which means the OS
> should not allow write access. The MTD partition binding supports this
> with the "read-only" property.

Yes, if the backing device has the capability to hw write protect
regions the exported fields overlap those then it does get ugly.
The MTD partition property was the inspiration here so perhaps it's
best to term this as a property indicating how the data region is
used in an implementation.

If it's left as a device property, then a binding with this property
would need to be defined for the Efuse, etc. cases..or a simple-nvmem
binding to handle the various OTP technologies.

> > The framework sysfs attributes provide a userspace EEPROM consumer where
> > it would be useful information to know that a data provider region is
> > read only rather than having the exported writeable attribute simply
> > fail a write cycle. This would allow the consumer to be aware that a
> > failed write (if even attempted) is expected if the data provider
> > advertised itself as read-only.
> 
> You could distinguish with RW versus RO file attributes.

Right, that would be preferred, based on the what the data provider
advertises.

-Matt
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