linux-kernel - Re: [PATCH v6 2/2] mfd: atmel-flexcom: add a driver for Atmel Flexible Serial Communication Unit

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Message-ID: <55B1195F.4000104@atmel.com>
Date:	Thu, 23 Jul 2015 18:42:07 +0200
From:	Cyrille Pitchen <cyrille.pitchen@...el.com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Lee Jones <lee.jones@...aro.org>
CC:	<nicolas.ferre@...el.com>, <alexandre.belloni@...e-electrons.com>,
	<sameo@...ux.intel.com>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <robh+dt@...nel.org>,
	<pawel.moll@....com>, <mark.rutland@....com>,
	<ijc+devicetree@...lion.org.uk>, <galak@...eaurora.org>,
	<devicetree@...r.kernel.org>
Subject: Re: [PATCH v6 2/2] mfd: atmel-flexcom: add a driver for Atmel Flexible
 Serial Communication Unit

Hi all,

Le 23/07/2015 14:50, Boris Brezillon a écrit :
> On Thu, 23 Jul 2015 10:13:11 +0100
> Lee Jones <lee.jones@...aro.org> wrote:
> 
>> On Thu, 23 Jul 2015, Boris Brezillon wrote:
>>
>>> Hi Lee,
>>>
>>> On Thu, 23 Jul 2015 08:32:17 +0100
>>> Lee Jones <lee.jones@...aro.org> wrote:
>>>
>>>> On Wed, 22 Jul 2015, Cyrille Pitchen wrote:
>>>>> +	for_each_child_of_node(np, child) {
>>>>> +		const char *compatible;
>>>>> +		int cplen;
>>>>> +
>>>>> +		if (!of_device_is_available(child))
>>>>> +			continue;
>>>>> +
>>>>> +		compatible = of_get_property(child, "compatible", &cplen);
>>>>> +		if (!compatible || strlen(compatible) > cplen)
>>>>> +			continue;
>>>>> +
>>>>> +		if (strstr(compatible, "-usart")) {
>>>>> +			opmode = FLEX_MR_OPMODE_USART;
>>>>> +			break;
>>>>> +		}
>>>>> +
>>>>> +		if (strstr(compatible, "-spi")) {
>>>>> +			opmode = FLEX_MR_OPMODE_SPI;
>>>>> +			break;
>>>>> +		}
>>>>> +
>>>>> +		if (strstr(compatible, "-i2c")) {
>>>>> +			opmode = FLEX_MR_OPMODE_TWI;
>>>>> +			break;
>>>>> +		}
>>>>> +	}
>>>>
>>>> From what I understand Flexcom is a wrapper which can sit above any
>>>> number of SPI, I2C and/or UART devices.  Devices which you don't
>>>> really have any control over (source code wise).  So wouldn't it be
>>>> better to match on the details you do have control over i.e. the node
>>>> name, rather than the compatible string?
>>>>
>>>> I would personally match on of_find_node_by_name() to future-proof
>>>> your implementation.
>>>
>>> Actually, I think using compatible strings is more future-proof than
>>> using the node names, because nothing in the DT bindings doc enforce the
>>> node name, and usually what we use to attach a node to a specific
>>> driver is the compatible string (this one is specified in the bindings
>>> doc).
>>
>> I know what you're saying, but what if someone uses the Flexcom driver
>> to wrap a different type of SPI driver where (for instance) the
>> compatible string used is "<name>-<newtype>".  Then we'd have to keep
>> adding more lines here to accommodate.
>>
>> Whereas if we used the child node name which only pertains to _this_
>> driver, we would then have full control and know that (unless it
>> Flexcom is used for a completely different type of serial controller)
>> we wouldn't have to keep expanding the code to accommodate.
> 
> You're right about the complexity implied by the compat string
> maintenance, but I still think using node names to detect the mode is
> a bad idea.
> 
> Let's take another example making both solution unsuitable: what if
> the flexcom-v2 exposes 2 devices of the same type, they will both have
> the same name and the same compatible string, and we'll have no way to
> detect the appropriate mode. That's why I think none of our suggestion
> is future-proof.
> 
>>
>>> Regarding the implementation itself, I would match the child node with
>>> an of_device_id table rather than trying to find a specific substring
>>> in the compatible string, but I think that's only a matter of taste.
>>
>> You mean duplicate each of the supported device's compatible strings
>> in this driver, then fetch the attributed of_match_device()->data
>> value?
>>
> 
> Yes, and that's definitely not a good idea, but I think Cyrille has
> found a better approach (I'll let him explain).

Indeed, what about taking advantage of the "ranges" property?

For the Flexcom:
#address-cells = <2>;
#size-cells = <1>;
ranges = <1 0 0xf8034200 0x200    /* opmode 1: USART */
          2 0 0xf8034400 0x200    /* opmode 2: SPI */
          3 0 0xf8034600 0x200>;  /* opmode 3: I2C */

Then for the single available child (for instance the SPI controller):
reg = <2 0 0x200>;

So the Operating Mode to be set into the Flexcom Mode Register is read from
the very first u32 of the "reg" property of the child.

No need to introduce any new DT property and the mapping remains easy to
maintain to follow hardware upgrades.

More details in v7 series.

> 
> Best Regards,
> 
> Boris
> 

Best Regards,

Cyrille
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