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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
From:   Frank Rowand <frowand.list@gmail.com>
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>
 <20200622151054.GW954398@dell>
 <037c0fd2-df35-5981-7ef2-c6199841650d@gmail.com>
Message-ID: <17457166-dfb6-34ee-3617-61a3a424ea00@gmail.com>
Date:   Mon, 22 Jun 2020 13:04:42 -0500
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On 2020-06-22 13:01, Frank Rowand wrote:
> On 2020-06-22 10:10, Lee Jones wrote:
>> On Mon, 22 Jun 2020, 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.
>>
>> I can see *what* you're trying to do.  I was looking for an
>> explanation of *how* you think that will work.  FWIW, I don't think
>> what you're proposing will work as you envisage.  I thought that
>> perhaps I was missing something, which is why I requested further
>> explanation.
>>
>>>>> 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.
>>
>> The problem is, how can you tell the difference between a valid and a
>> broken FDT without pre-processing - which, as I explained in the
>> commit message, I am not prepared to do.  We cannot test individually
>> since all configurations (e.g. no 'reg' property are valid on an
>> individual basis.
> 
> If my proposed changes are made, then there are at least 3 ways to detect
> a broken FDT or prevent the problem caused by the broken FDT.
> 
> 
> 1) Use the validation process that uses the bindings to validate the
> devicetree source.
> 
> 
> 2) Modify patch 1/3.  The small part of the patch to modify is:
> 
> +static int mfd_match_of_node_to_dev(struct platform_device *pdev,
> +				    struct device_node *np,
> +				    const struct mfd_cell *cell)
> +{
> +	struct mfd_of_node_entry *of_entry;
> +	const __be32 *reg;
> +	u64 of_node_addr;
> +
> +	/* Skip devices 'disabled' by Device Tree */
> +	if (!of_device_is_available(np))
> +		return -ENODEV;
> +
> +	/* Skip if OF node has previously been allocated to a device */
> +	list_for_each_entry(of_entry, &mfd_of_node_list, list)
> 
> Change:
> 
> +		if (of_entry->np == np)
> +			return -EAGAIN;
> 
> To:
> 
> +		if (of_entry->np == np) {
> +			if (!cell->use_of_reg)
> +				return -EINVAL;
> +			else
> +				return -EAGAIN;
> 
> There may be a better choice than EINVAL, but I am just showing the method.
> 
> You may also want to refactor this section of the patch slightly
> differently to achieve the same result.  It was just easiest to
> show the suggested change the way I did it.
> 
> The test that returns EINVAL detects the issue that the FDT does
> not match the binding (there is more one child node with the
> "stericsson,ab8500-pwm" compatible.
> 
> 
> 3) I'm not sure if the pre-parsing that is wanted is parsing of the

                                     ^^^^^^^^^^^^^^ that is not wanted

> devicetree or parsing of the struct mfd_cell array.  If the mfd_cell
> array then solution 3 is not acceptable.
> 
> A different change to a small part of patch 1/3.  In mfd_add_devices(),
> validate parameter "cells".  The validation could precede the existing
> code, or it could be folded into the existing for loop.  The validation
> is checking for any other element of the cells array containing
> the same compatible value if cell->use_of_reg is not true for an element.
> 
> If this validation occurs, then I think mfd_of_node_list, and all the
> associated code to deal with it is no longer needed.  But I didn't
> look at this part in detail, so maybe I missed something.
> 
> The validation is something like (untested):
> 
> 	if (IS_ENABLED(CONFIG_OF)
> 		for (i = 0; i < n_devs; i++) {
> 			this_cell = cells + i;
> 			if (!this_cell->use_of_reg) {
> 				for (j = 1; j < n_devs; j++) {
> 					if (j != i) {
> 						cell = cells + j;
> 						if (!strcmp(this_cell->of_compatible, cell->of_compatible))
> 							return -EINVAL;
> 					}
> 				}
> 			}
> 		}
> 	
> 
> 
> 
>>
>> The best we can do is "best effort", to try and match each cell with
>> its requested OF node.
>>
>