linux-kernel - Re: [PATCH v5 1/4] i2c: busses: i2c-st: Add ST I2C controller

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Date:	Mon, 28 Oct 2013 16:02:36 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Srinivas KANDAGATLA <srinivas.kandagatla@...com>,
	Lucas Stach <l.stach@...gutronix.de>
Cc:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Russell King <linux@....linux.org.uk>,
	Pawel Moll <pawel.moll@....com>,
	Wolfram Sang <wsa@...-dreams.de>,
	Stephen Warren <swarren@...dotorg.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <rob.herring@...xeda.com>,
	Stephen GALLIMORE <stephen.gallimore@...com>,
	Stuart MENEFY <stuart.menefy@...com>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	Rob Landley <rob@...dley.net>,
	Grant Likely <grant.likely@...aro.org>,
	Lee Jones <lee.jones@...aro.org>,
	Gabriel FERNANDEZ <gabriel.fernandez@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 1/4] i2c: busses: i2c-st: Add ST I2C controller


On 10/18/2013 10:22 AM, Srinivas KANDAGATLA wrote:
> On 17/10/13 15:49, Lucas Stach wrote:
>> Am Donnerstag, den 17.10.2013, 15:30 +0100 schrieb srinivas kandagatla:
>> [...]
>>> Sorry to ask this but, Where is this requirement coming from?
>>> I have not spotted any thing as such in ePAPR specs.
>>>
>>>
>>> All the spec says is.
>>> ===
>>> The compatible property value consists of one or more strings that
>>> define the specific programming model for the device. This list of
>>> strings should be used by a client program for device driver selection.
>>> The property value consists of a concatenated list of null terminated
>>> strings, *from most specific to most general.* They allow a device to
>>> express its compatibility with a family of similar devices, potentially
>>> allowing a single device driver to match against several devices.
>>> The recommended format is “manufacturer,model”, where manufacturer is a
>>> string describing the name of the manufacturer (such as an OUI), and
>>> model specifies the model number.
>>>
>>> Example:
>>> compatible = “fsl,mpc8641-uart”, “ns16550";
>>> In this example, an operating system would first try to locate a device
>>> driver that supported fsl,mpc8641-uart. If a driver was not found, it
>>> would then try to locate a driver that supported the more general
>>> ns16550 device type.
>>> ===
>>>
>>> The more general compatible string in this case is "st,comms-ssc-i2c",
>>> rather than the first soc name.
>>> How can a first SOC name be more general?
>>>
>>> As this driver is not very specific to StiH415, it is generic driver for
>>> ST comms-ssc-i2c block.
>>>
>> You just can't know if someone in the future decides to subtly change
>> the register layout or make some other incompatible change to the
>> comms-ssc-i2c block.
>>
> This is not the case for comms-ssc-i2c block, This IP is kind of reused
> from past 10+ years(I think!!). Am not predicting the future here, but I
> am making a informed guess from past experience that this IP would not
> change in future.
Having discussed with HW design team in charge of this IP,
I also bet that this IP won't change in the future.

>
> Am still not happy with the idea of using first SoC for the compatible
> for following reasons:
>
> 1> Generic IPs can be integrated into various vendor SoCs. For example
> synopsis IP can be integrated by ST parts and other non-ST parts. What
> would be the first SoC name in this case?
>
> 2> Looking at example like "arm,pl310-cache", "arm,l220-cache"... or any
> other generic ips, why are these IPs not encoding the first SoC name in
> there compatible string? I think the answer is generic IP.
>
> 3> IMHO, the idea of first SoC might solve the problem you described,
> but why would some one know about the first SoC when this was available.
> In this case this IP was available may be 10+ years back on an ST40
> platform. Having such old SoC names in compatible strings in the device
> trees for a modern chip looks bit confusing.
>
> 4> ST generic drivers which are in kernel still use st,<IP> name, so I
> would like this consistency across all the st drivers (at least the ones
> which are going to be used by mach-sti platforms).
>
> 5> ePAPR spec clearly says that compatible string should contain "most
> specific to most general" names. In this case using more generic name
> makes more sense than having a specific name because its generic IP.
> Allowing a single device driver to match against several devices.
>
> 6> IMHO, the compatible string should be "vendor,<IP-name>-<IP-version>"
> rather than first SoC.
I agree.
In this case, we add support to revision 4 of SSC IP.

Is "st,comms-ssc-v4" okay?


Thanks,
Maxime
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