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 09:23:52 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Michael Walle <michael@...le.cc>
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

On Wed, 24 Jun 2020, Michael Walle wrote:

> 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.

The fix mentioned above is orthogonal to this set.

The *only* reason for the differing node names there is to circumvent
the following DTC warnings:

arch/arm/boot/dts/ste-ab8500.dtsi:210.16-214.7: ERROR (duplicate_node_names): /soc/prcmu@...57000/ab8500/ab8500-pwm: Duplicate node name
arch/arm/boot/dts/ste-ab8500.dtsi:216.16-220.7: ERROR (duplicate_node_names): /soc/prcmu@...57000/ab8500/ab8500-pwm: Duplicate node name
arch/arm/boot/dts/ste-ab8500.dtsi:216.16-220.7: ERROR (duplicate_node_names): /soc/prcmu@...57000/ab8500/ab8500-pwm: Duplicate 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.

I don't have a strong opinion either way.

We can *also* add node name matching if Rob deems it fit.

-- 
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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