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Subject: Re: [PATCH v2 1/3] mfd: core: Make a best effort attempt to match
 devices with the correct of_nodes
To:     Lee Jones <lee.jones@linaro.org>
Cc:     andy.shevchenko@gmail.com, michael@walle.cc, robh+dt@kernel.org,
        broonie@kernel.org, devicetree@vger.kernel.org,
        linus.walleij@linaro.org, linux@roeck-us.net,
        andriy.shevchenko@linux.intel.com, robin.murphy@arm.com,
        gregkh@linuxfoundation.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, Frank Rowand <frowand.list@gmail.com>
References: <20200611191002.2256570-1-lee.jones@linaro.org>
 <4b188fb5-6667-720d-46e1-6f103efe8966@gmail.com>
 <20200615092644.GA2608702@dell>
 <eef50a78-8571-5600-4fee-c824fd4a7f69@gmail.com>
 <20200622085009.GP954398@dell>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <cd8952da-cc55-8087-b9f6-876417beb188@gmail.com>
Date:   Mon, 22 Jun 2020 09:32:07 -0500
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On 2020-06-22 03:50, Lee Jones wrote:
> On Thu, 18 Jun 2020, Frank Rowand wrote:
> 
>> 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.
> 
> Sorry, but I'm not sure what the above exercise is supposed to solve.
> 
> Could you explain it for me please?

The OF_MFD_CELL() entry handles all of the existing .dts source files
that only have one ab8500-pwm child nodes.  So existing .dtb blobs
continue to work.

The OF_MFD_CELL_REG() entries will handle all of the new .dts source
files that will have up to 3 ab8500-pwm child nodes.

Compatibility is maintained for existing .dtb files.  A new kernel
version with the changes will support new .dtb files that contain
multiple ab8500-pwm child nodes.

> 
>> 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.
> 
> What fallback code?

Based on reading the patch description, I expected some extra code to try
to handle the case where the compatible in more than one struct mfd_cell
entry is "stericsson,ab8500-pwm" and there are multiple ab8500-pwm child
nodes.

Looking at the actual code (which I had not done before), I see that the
"best effort attempt to match" is keeping a list of child nodes that
have already been used (mfd_of_node_list) and avoiding re-use of such
nodes.  This allows an invalid .dtb (one with multple "stericsson,ab8500-pwm"
child nodes) to possibly be assigned unique child nodes for multiple
struct mfd_cell entries to be "stericsson,ab8500-pwm".

So it is confusing for me to call that "fallback code".  It really is
"best effort attempt to match" for a broken .dtb code.

There should be no best effort for a broken .dtb.  The broken .dtb should
instead result in an error.

-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.
>>>
>>
>