Re: [PATCH v4 05/10] eeprom: Add bindings for simple eeprom framework

[Srinivas Kandagatla <srinivas.kandagatla@linaro.org>]

On 07/04/15 20:46, Matt Porter wrote:
> On Tue, Apr 07, 2015 at 07:03:30PM +0100, Srinivas Kandagatla wrote:
>>
>>
>> On 07/04/15 18:46, Mark Brown wrote:
>>> On Tue, Apr 07, 2015 at 06:35:49PM +0100, Srinivas Kandagatla wrote:
>>>> On 06/04/15 16:04, Matt Porter wrote:
>>>>> On Mon, Apr 06, 2015 at 09:11:05AM -0500, Rob Herring wrote:
>>>
>>>>>>> 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
>>>
>>>> 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.
>>>
>>> There is the ability to flag read/write permissions in regmap but I
>>> think there's some suggestion that this should be exposed to userspace
>>> so that it's easier for it to handle things rather than just writing
>>> then coping with any errors.
>>
>> Yes, That's possible if the data provider use the "read-only" generic
>> binding like MTD partitions which the eeprom framwork could use to set the
>> binary file mode appropriately.
>
> Right, that's what Rob suggested as to how it should be exposed to
> userspace. I think Mark is suggesting that it can also be done by
> returning appropriately fine-grained error codes from a writeable
> attribute.
>


We are taking about two things here.

1: "Setting the correct mode for the user space binary file."
	This is only possible if the entire eeprom/nvmem has write protection. 
We could get that info from 2 places. One having a new DT property 
bindings like "read-only" as Rob suggested.
	or
	Second, scan the regmap for writeable attributes and the correct file 
mode. For this option AFAIK, with existing regmap code we can only do 
this by scanning the entire range, which is bad I guess.

2: "Returning appropriate error codes to user space."
	This is already addressed in the existing code, regmap would return an 
(-EIO) I/O error Or error codes from providers in cases where an write 
attempt to non writable register/or something wrong is made and the 
*same error* code is passed back to user too. All the error codes from 
regmap/provider layer are always passed back to the user space. These 
error code, I supposed are fine grained originating from the low layer.


I think with "read-only" property and returning same error codes from 
regmap/provider layer to user-space would address the points raised by Matt.


> Just to clarify here, I brought this up because if we only allow
> the writes to fail, there's not necessarily not enough information
> available to know if they failed due to a real error (perhaps write
> cycles for the underlying nvmem device have been exhausted) or is
> it simply that the underlying device has been hardware write protected
> (such as as the write protect pin hardwired that way or it's an OTP
> device). The client, whether userspace or otherwise is going to need
> to know this information to make informed policy decisions.
>


Thanks for the inputs.


The exiting regmap writeable_register api will only return true or 
false. But if the provider implements its own read/writes functions, the 
error-codes from these apis would be passed back to user anyway, so the 
user can make informed policy decisions. This logic already present in 
with the exiting eeprom/regmap code.



--srini
> -Matt
>
>> "read-only" property seems to be more generic for all types of data
>> providers.
>>
>> I will give it a try and document this in the bindings too in next version.
>>
>> --srini
>>>