Re: [PATCH 13/18] iio: adc: mcp320x: Set struct spi_driver .of_match_table

[Javier Martinez Canillas]

Hello Michael,

On 08/21/2015 12:29 AM, Michael Welling wrote:
> On Fri, Aug 21, 2015 at 12:02:40AM +0200, Javier Martinez Canillas wrote:
>> Hello Michael,
>>
>> On 08/20/2015 10:09 PM, Michael Welling wrote:
>>> On Thu, Aug 20, 2015 at 09:07:26AM +0200, Javier Martinez Canillas wrote:
>>>> The driver has an OF id table but the .of_match_table is not set so
>>>> the SPI core can't do an OF style match and the table was unused.
>>>>
>>>
>>> Is an OF style match necessary?
>>>
>>
>> Did you read the cover letter [0] on which I explain why is needed to
>> avoid breaking module autoloading in the future? Once the SPI core is
>> changed by RFC patch 18/18? (you were cc'ed in the cover letter BTW).
> 
> Well I have read it now. :)
>

Great :)
 
>>
>>> I have been using devicetree and it matches based on the .id_table.
>>>
>>
>> Yes it fallbacks to the .id_table or the driver name but the correct
>> thing to do for devices registered by OF, is to match using the
>> compatible string.
>>
>>> Couldn't we just remove the mcp320x_dt_ids table instead?
>>>
>>
>> No, that is the wrong thing to do IMHO since the compatible string
>> contains both vendor and device name whle the .id_table only contains
>> a device name.
>>
>> So it makes sense to match using the compatible string and also report
>> the OF modalias information to user-space.
>>
>> Otherwise what's the point of the vendor in the compatible string for
>> SPI devices? You can just use "bar" instead of "foo,bar" as a string.
>>
> 
> Well then shouldn't the patch include adding the vendor to the compatible
> string?
>

Well, I was talking in general. You are right that this specific driver does
not have a vendor prefix for the compatible strings. This is incorrect
according to the ePAPR document [0].

However, these compatible strings are already documented as a DT binding doc
in Documentation/devicetree/bindings/iio/adc/mcp320x.txt so I don't know
what is the correct thing to do in this situation.

Changing the compatible string will break old DTB with newer kernels and that
is a no go. But that doesn't invalidate what I said since once the SPI core
is changed and report OF modalias, you will need a MODULE_DEVICE_TABLE(of,...)
regardless if the compatible string has a vendor prefix or not.

[0]: https://www.power.org/documentation/epapr-version-1-1/

Best regards,
-- 
Javier Martinez Canillas
Open Source Group
Samsung Research America
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