Re: [PATCH v1] mtd: parsers: ofpart: Fix parsing when size-cells is 0

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Marek,

marex@denx.de wrote on Thu, 15 Dec 2022 08:45:33 +0100:

> On 12/15/22 08:16, Miquel Raynal wrote:
> > Hi Marek & Francesco,  
> 
> Hi,
> 
> > marex@denx.de wrote on Mon, 5 Dec 2022 17:25:11 +0100:
> >   
> >> On 12/5/22 14:49, Miquel Raynal wrote:  
> >>> Hi Francesco,  
> >>
> >> Hi,
> >>  
> >>> francesco@dolcini.it wrote on Mon, 5 Dec 2022 12:26:44 +0100:  
> >>>    >>>> On Fri, Dec 02, 2022 at 06:08:22PM +0100, Marek Vasut wrote:  
> >>>>> But here I would say this is a firmware bug and it might have to be handled
> >>>>> like a firmware bug, i.e. with fixup in the partition parser. I seem to be
> >>>>> changing my opinion here again.  
> >>>>
> >>>> I was thinking at this over the weekend, and I came to the following
> >>>> ideas:
> >>>>
> >>>>    - we need some improvement on the fixup we already have in the
> >>>>      partition parser. We cannot ignore the fdt produced by U-Boot - as
> >>>>      bad as it is.
> >>>>    - the proposed fixup is fine for the immediate need, but it is
> >>>>      not going to be enough to cover the general issue with the U-Boot
> >>>>      generated partitions. U-Boot might keep generating partitions as direct
> >>>>      child of the nand controller even when a partitions{} node is
> >>>>      available. In this case the current parser just fails since it looks
> >>>>      only into it and it will find it empty.
> >>>>    - the current U-Boot only handle partitions{} as a direct child of the
> >>>>      nand-controller, the nand-chip is ignored. This is not the way it is
> >>>>      supposed to work. U-Boot code would need to be improved.  
> >>>
> >>> I've been thinking about it this weekend as well and the current fix
> >>> which "just set" s_cell to 1 seems risky for me, it is typically the
> >>> type of quick & dirty fix that might even break other board (nobody
> >>> knew that U-Boot current logic expected #size-cells to be set in the
> >>> DT, what if another "broken" DT expects the opposite...)  
> >>
> >> Then with the current configuration, such broken DT would not work, since current DT does set #size-cells=<1> (wrongly).
> >>  
> >>> , not
> >>> mentioning potential issues with big storages (> 4GiB).
> >>>
> >>> All in all, I really think we should revert the DT change now, reverting
> >>> as little to no drawbacks besides a dt_binding_check warning and gives
> >>> us time to deal with it properly (both in U-Boot and Linux).  
> >>
> >> I am really not happy with this, but if that's marked as intermediate fix, go for it.
> >>
> >> How do we deal with this in the long run however? Parser-side fix like this one, maybe with better heuristics ?  
> > 
> > Yesterday while talking about an ACPI mis-description which needed
> > fixing, I realized fixing up what the firmware provides to Linux should
> > preferably be handled as early as possible. So my first first idea was
> > to avoid using the broken "fixup mtdparts" function in U-Boot and I am
> > still convinced this is what we should do in priority. However, as
> > rightly pointed in this thread, we need to take care about the case
> > where someone would use a newer DT (let's say, with the reverted changed
> > reverted again) with an old U-Boot. I am still against piggy hacks in
> > the generic ofpart.c driver, but what we could do however is a DT
> > fixup in the init_machine (or the dt_fixup) hook for imx7 Colibri, very
> > much like this:
> > https://elixir.bootlin.com/linux/latest/source/arch/arm/mach-mvebu/board-v7.c#L111
> > Plus a warning there saying "your dt is broken, update your firmware".  
> 
> This does not work, because the old U-Boot fixup_mtdparts() may be applied on any machine,

No: https://elixir.bootlin.com/u-boot/latest/A/ident/fdt_fixup_mtdparts
And we should make our best so its use does not proliferate.
It's not like there is half a dozen of good ways to describe and forward
partitions today.

> it is not colibri mx7 specific. Also, new arch-side workaround are
> really not welcome by the architecture maintainers as far as I can
> tell.

So what? Let's propose the change and see what the maintainers have to
say. I am open to discussion.

As I said, it is not colibri mx7 specific, there are a few boards which
might be affected, they are all clearly identifiable with a compatible.
It's not the entire planet either.

> > So next time someone stumbles upon this issue, we can tell them "fix
> > your bootloader", and apply the same hack in their board family (there
> > are three or four IIRC which might be concerned some day).  
> 
> There are also those machines we do not even know about which might be generating bogus DT using old U-Boot and fixup_mtdparts(), so, unless there is some all-arch fixup implementation, we wouldn't be able to fix them all on arch side. I think the all-arch fixup implementation would be the driver one, i.e. this patch as it is (or maybe with some improvement).

If we don't know about them, as you say, I don't feel concerned.

If something is buggy, people will report it, we will point them in the
right direction so they can fix their firmware and propose a similar
fix in their case which will involve adding a new machine compatible to
the list of boards that should tweak the #size-cell property.

> > That would fix all cases and only have an impact on the affected boards.  
> 
> Sadly, it does only fix the known cases, not the unknown cases like downstream forks which never get any bootloader updates ever, and which you can't find in upstream U-Boot, and which you therefore cannot easily catch in the arch side fixup.

And ?

Thanks,
Miquèl

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