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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
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 <20200622085009.GP954398@dell>
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Date:   Mon, 22 Jun 2020 20:37:59 -0500
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On 2020-06-22 20:17, Frank Rowand wrote:
> On 2020-06-22 17:23, Frank Rowand wrote:
>> On 2020-06-22 14:11, Lee Jones wrote:
>>> On Mon, 22 Jun 2020, 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 "".
>>>>>>
>>>>>> 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.
>>>
>>> Could you provide an example please?
>>
>> I'm sorry I don't have a concise description and example.  I have not been
>> keeping up with the state of the art in this area.
>>
>> A terribly non-precise pointer to the general area is:
>>
>>   https://elinux.org/Device_tree_future#Devicetree_Verification
> 
> I haven't had time to search for an excellent resource yet, but following
> the above URL, the is a Linaro Connect slide set from Grant Likely that
> provides a somewhat high level conceptual view.  You can skim through
> the slides pretty fast:
> 
>    https://elinux.org/images/6/67/Hkg18-120-devicetreeschema-grantlikely-180404144834.pdf

A better presentation for this purpose is Rob's:

  https://elinux.org/images/6/6b/LPC2018_json-schema_for_Devicetree.pdf

-Frank

> 
> A very high level overview is that the bindings documents in the Linux
> kernel source tree at Documentation/devicetree/bindings/ are being
> converted to a YAML format that can be processed by the verification
> tools.  The verification tools can use the bindings to check whether
> a devicetree source follows the definition of the bindings for each
> of the nodes.
> 
> A random example of a binding that has been converted to YAML is
> 
>    Documentation/devicetree/bindings/rng/st,stm32-rng.yaml
> 
> In the specific case that this patch series addresses, the
> Documentation/devicetree/bindings/mfd/ab8500.txt binding has not been
> converted to YAML yet.  If it was YAML, it should specify the properties
> for a child node with compatible value "stericsson,ab8500-pwm" is not
> allowed to have a "reg" property.  A new compatible should be
> added for child nodes that are required to have a "reg" property.
> In my suggestion above I chose "ab8500-pwm-mc" for this new compatible.
> That is probably a terrible name, Rob would probably have a better
> suggestion.
> 
> Given such a YAML binding, the verification tool would report an error
> for any child node with compatible "stericsson,ab8500-pwm" and containing
> a "reg" property.  It would also report an error for any child node
> with compatible "ab8500-pwm-mc" that was missing the required "reg"
> property.
> 
> -Frank
> 
>>
>> As a general comment, I think that validation / verification is a very
>> valuable tool, but the schemas to support it are an ongoing project.
>>
>> Even after the schemas are all in place, it will still be possible for
>> bad FDTs to be fed to the kernel, so it is not a total pancea.
>>
>>>
>>>> 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.
>>>
>>> So here, instead of just failing a single device, we fail everything?
>>> Sounds a lot like throwing the baby out with the bath water.  How is
>>> that an improvement?
> 
> You can modify more extensively than my simple example, changing
> mfd_add_device() to more gracefully handle an EINVAL returned by
> mfd_match_of_node_to_dev().
> 
> 
>>>
>>>> 3) I'm not sure if the pre-parsing that is wanted is parsing of the
>>>> 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;
>>>> 					}
>>>> 				}
>>>> 			}
>>>> 		}
>>>
>>> I think I just threw-up a little. ;)
>>
>> I'm not surprised.
>>
>> But it actually is pretty simple code.
>>
>>>
>>> Did you read the commit message?
>>
>> Yes, I did.
>>
>>>
>>>   "We could code around this with some pre-parsing semantics, but the
>>
>> And as I said above, it was not clear to me what was meant by pre-parsing.
>>
>>>   added complexity required to cover each and every corner-case is not
>>>   justified.  Merely patching the current failing (via this patch) is
>>>   already working with some pretty small corner-cases"
>>>   
>>> Providing thorough pre-parsing would be highly complex and highly
>>> error prone.  The example you provide above is not only ugly, there
>>> are numerous issues with it.  Not least:
>>>
>>>  * Only one corner-case is covered
>>
>> I agree with this.  I also agree it is a fool's errand to try to add
>> code to fully validate all possible devicetree source errors in
>> driver source.
>>
>>>  * Validation is only completed on a single mfd_cells struct
>>
>> On a single _array_ of struct mfd_cells.  But this does appear
>> to be a fatal flaw.  I had not looked at enough callers of
>> mfd_add_devices() to see that it is a common pattern for
>> a single driver to call it multiple times.
>>
>>>  * False positives can occur and will fail as a result
>>
>> ((What is an example of a false positive?))  Never mind, now that
>> I see that the previous issue is a fatal flaw, this becomes
>> academic.
>>
>>>
>>> The above actually makes the solution worse, not better.
>>>
>>
>> Patch 1/3 silently fails to deal with a broken devicetree.
>> It results on one of the three ab8500-pwm child nodes in
>> the hypothetical devicetree source tree not being added.
>>
>> That is not a good result either.
>>
>> OK, so my solution #3 is a no go.  How about my solution #2,
>> which you did not comment on?
>>
>> -Frank
>> .
>>
> 
> .
>