Message-ID: <55241575.7040809@linaro.org>
Date:	Tue, 07 Apr 2015 18:35:49 +0100
From:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:	Matt Porter <mporter@...sulko.com>,
	Rob Herring <robherring2@...il.com>
CC:	"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

Thanks Matt and Rob for review,

On 06/04/15 16:04, Matt Porter wrote:
> 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".
>>>


nvmem sounds sensible name, I will rename framework to nvmem in next 
version.


>>>> +
>>>> +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.
>
Correct me If am wrong.

Regarding write protection/read-only, regmap already has provisions to 
support this feature. regmap would bail out with errors if any attempt 
to write to non-writable regions. It all depends on the data providers 
how they setup the regmap and the bindings for those are specific 
individual data providers I think.

This would protect the user space side and kernel side consumers as well.

This should address your original query, I guess :-)


Thanks,
srini

> 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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