Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
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
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>
 <cd8952da-cc55-8087-b9f6-876417beb188@gmail.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <f1e03639-a202-ab91-df38-453320d1ddeb@gmail.com>
Date:   Mon, 22 Jun 2020 09:35:50 -0500
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.8.0
MIME-Version: 1.0
In-Reply-To: <cd8952da-cc55-8087-b9f6-876417beb188@gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 2020-06-22 09:32, Frank Rowand wrote:
> 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".

  struct mfd_cell entries that each have the same compatible value
  "stericsson,ab8500-pwm".

Some day I'll learn how to speak my native language. :-)

-Frank

> 
> 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.
>>>>
>>>
>>
>