linux-kernel - Re: [PATCH 13/18] iio: adc: mcp320x: Set struct spi_driver .of_match

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Date:	Sat, 22 Aug 2015 18:48:53 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Javier Martinez Canillas <javier@....samsung.com>,
	Michael Welling <mwelling@...e.org>
Cc:	linux-kernel@...r.kernel.org, Andrea Galbusera <gizero@...il.com>,
	linux-iio@...r.kernel.org, Peter Meerwald <pmeerw@...erw.net>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Manfred Schlaegl <manfred.schlaegl@....at>,
	Søren Andersen <san@...etechnology.dk>,
	Hartmut Knaack <knaack.h@....de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 13/18] iio: adc: mcp320x: Set struct spi_driver
 .of_match_table

On 20/08/15 23:48, Javier Martinez Canillas wrote:
> 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.
Take the view the old version is wrong but needs to be supported and add also
the corrected strings + document them.

cc'd The device tree list for any more comments on this.
> 
> 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,
> 

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