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:   Thu, 18 Jun 2020 12:34:07 -0500
From:   Frank Rowand <frowand.list@...il.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     andy.shevchenko@...il.com, michael@...le.cc, 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 2020-06-15 04:26, Lee Jones wrote:
> On Sun, 14 Jun 2020, Frank Rowand wrote:
> 
>> Hi Lee,
>>
>> I'm looking at 5.8-rc1.
>>
>> The only use of OF_MFD_CELL() where the same compatible is specified
>> for multiple elements of a struct mfd_cell array is for compatible
>> "stericsson,ab8500-pwm" in drivers/mfd/ab8500-core.c:
>>
>>         OF_MFD_CELL("ab8500-pwm",
>>                     NULL, NULL, 0, 1, "stericsson,ab8500-pwm"),
>>         OF_MFD_CELL("ab8500-pwm",
>>                     NULL, NULL, 0, 2, "stericsson,ab8500-pwm"),
>>         OF_MFD_CELL("ab8500-pwm",
>>                     NULL, NULL, 0, 3, "stericsson,ab8500-pwm"),

         OF_MFD_CELL("ab8500-pwm",
                     NULL, NULL, 0, 0, "stericsson,ab8500-pwm"),

         OF_MFD_CELL_REG("ab8500-pwm-mc",
                         NULL, NULL, 0, 0, "stericsson,ab8500-pwm", 0),
         OF_MFD_CELL_REG("ab8500-pwm-mc",
                         NULL, NULL, 0, 1, "stericsson,ab8500-pwm", 1),
         OF_MFD_CELL_REG("ab8500-pwm-mc",
                         NULL, NULL, 0, 2, "stericsson,ab8500-pwm", 2),

>>
>> The only .dts or .dtsi files where I see compatible "stericsson,ab8500-pwm"
>> are:
>>
>>    arch/arm/boot/dts/ste-ab8500.dtsi
>>    arch/arm/boot/dts/ste-ab8505.dtsi
>>
>> These two .dtsi files only have a single node with this compatible.
>> Chasing back to .dts and .dtsi files that include these two .dtsi
>> files, I see no case where there are multiple nodes with this
>> compatible.
>>
>> So it looks to me like there is no .dts in mainline that is providing
>> the three "stericsson,ab8500-pwm" nodes that drivers/mfd/ab8500-core.c
>> is expecting.  No case that there are multiple mfd child nodes where
>> mfd_add_device() would assign the first of n child nodes with the
>> same compatible to multiple devices.
>>
>> So it appears to me that drivers/mfd/ab8500-core.c is currently broken.
>> Am I missing something here?
>>
>> If I am correct, then either drivers/mfd/ab8500-core.c or
>> ste-ab8500.dtsi and ste-ab8505.dtsi need to be fixed.
> 
> Your analysis is correct.

OK, if I'm not overlooking anything, that is good news.

Existing .dts source files only have one "ab8500-pwm" child.  They already
work correcly.

Create a new compatible for the case of multiple children.  In my example
I will add "-mc" (multiple children) to the existing compatible.  There
is likely a better name, but this lets me provide an example.

Modify drivers/mfd/ab8500-core.c to use the new compatible, and new .dts
source files with multiple children use the new compatible:

         OF_MFD_CELL("ab8500-pwm",
                     NULL, NULL, 0, 0, "stericsson,ab8500-pwm"),

         OF_MFD_CELL_REG("ab8500-pwm-mc",
                         NULL, NULL, 0, 0, "stericsson,ab8500-pwm", 0),
         OF_MFD_CELL_REG("ab8500-pwm-mc",
                         NULL, NULL, 0, 1, "stericsson,ab8500-pwm", 1),
         OF_MFD_CELL_REG("ab8500-pwm-mc",
                         NULL, NULL, 0, 2, "stericsson,ab8500-pwm", 2),

The "OF_MFD_CELL" entry is the existing entry, which will handle current
.dts source files.  The new "OF_MFD_CELL_REG" entries will handle new
.dts source files.

And of course the patch that creates OF_MFD_CELL_REG() needs to precede
this change.

I would remove the fallback code in the existing patch that tries to
handle an incorrect binding.  Just error out if the binding is not
used properly.

-Frank

> 
> Although it's not "broken", it just works when it really shouldn't.
> 
> I will be fixing the 'ab8500-pwm' case in due course.
> 
>> Moving forward, your proposed OF_MFD_CELL_REG() method seems a good
>> approach (I have not completely read the actual code in the patch yet
>> though).
> 
> Thanks.
>