linux-kernel - Re: [PATCH v2 1/3] mfd: core: Make a best effort attempt to match devices with the correct of

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Date:   Wed, 24 Jun 2020 09:47:20 +0200
From:   Michael Walle <michael@...le.cc>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     Frank Rowand <frowand.list@...il.com>, andy.shevchenko@...il.com,
        robh+dt@...nel.org, broonie@...nel.org, devicetree@...r.kernel.org,
        linus.walleij@...aro.org, linux@...ck-us.net,
        andriy.shevchenko@...ux.intel.com, robin.murphy@....com,
        gregkh@...uxfoundation.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] mfd: core: Make a best effort attempt to match
 devices with the correct of_nodes

Hi,

Am 2020-06-24 08:41, schrieb Lee Jones:
> On Tue, 23 Jun 2020, Frank Rowand wrote:
> 
>> On 2020-06-11 14:10, Lee Jones wrote:
>> > Currently, when a child platform device (sometimes referred to as a
>> > sub-device) is registered via the Multi-Functional Device (MFD) API,
>> > the framework attempts to match the newly registered platform device
>> > with its associated Device Tree (OF) node.  Until now, the device has
>> > been allocated the first node found with an identical OF compatible
>> > string.  Unfortunately, if there are, say for example '3' devices
>> > which are to be handled by the same driver and therefore have the same
>> > compatible string, each of them will be allocated a pointer to the
>> > *first* node.
>> 
>> As you mentioned elsewhere in this thread, this series "fixes" the
>> problem related to the "stericsson,ab8500-pwm" compatible.
>> 
>> I know, I said I would drop discussion of that compatible, but bear
>> with me for a second.  :-)
>> 
>> The "problem" is that the devices for multiple mfd child nodes with
>> the same compatible value of "stericsson,ab8500-pwm" will all have
>> a pointer to the first child node.  At the moment the same child
>> of_node being used by more than one device does not cause any
>> incorrect behavior.
>> 
>> Just in case the driver for "stericsson,ab8500-pwm" is modified
>> in a way that the child of_node needs to be distinct for each
>> device, and that changes gets back ported, it would be useful
>> to have Fixes: tags for this patch series.
>> 
>> So, at your discretion (and I'll let you worry about the correct
>> Fixes: tag format), this series fixes:
>> 
>> bad76991d7847b7877ae797cc79745d82ffd9120 mfd: Register ab8500 devices 
>> using the newly DT:ed MFD API
> 
> This patch isn't actually broken.
> 
> The issue is the DTB, which [0] addresses.
> 
> [0]
> https://lkml.kernel.org/lkml/20200622083432.1491715-1-lee.jones@linaro.org/

Now, I'm confused; because this patch doesn't use the reg property
but a different node name. I'd actually prefer this for any MFD
driver which has multiple nodes of the same compatible string. See
my reasoning here [1]. But until now, no one has responded. Thus,
I'd rather see a OF_MFD_CELL_NAME() which matches the node name
instead of the OF_MFD_CELL_REG() macro.

This would also circumvent the fact that the unit-address has one
number space. Eg. it is not possible to have:

mfd {
   compatible = "mfd,compatible";

   gpio@0 {
     reg = <0>;
   };
   gpio@1 {
     reg = <1>;
   };
   pwm@0 {
     reg = <0>;
   };
};

Although Rob mentioned to maybe relax that, but I sill fail to see
the advantage to have an arbitrary reg property instead of a unique
node name.

[1] 
https://lore.kernel.org/linux-devicetree/0709f20bc61afb6656bc57312eb69f56@walle.cc/

-- 
-michael