[PATCH v2 1/3] mfd: core: Make a best effort attempt to match devices with the correct of_nodes
Lee Jones
lee.jones at linaro.org
Mon Jun 22 15:11:33 EDT 2020
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 "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.
Could you provide an example please?
> 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?
> 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. ;)
Did you read the commit message?
"We could code around this with some pre-parsing semantics, but the
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
* Validation is only completed on a single mfd_cells struct
* False positives can occur and will fail as a result
The above actually makes the solution worse, not better.
--
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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