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 Fri, 2 Dec 2022 16:23:29 +0100:

> On 12/2/22 16:00, Miquel Raynal wrote:
> > Hi Marek,  
> 
> Hi,
> 
> > marex@denx.de wrote on Fri, 2 Dec 2022 15:31:40 +0100:
> >   
> >> On 12/2/22 15:05, Miquel Raynal wrote:  
> >>> Hi Francesco,  
> >>
> >> Hi,
> >>
> >> [...]
> >>  
> >>> I still strongly disagree with the initial proposal but what I think we
> >>> can do is:
> >>>
> >>> 1. To prevent future breakages:
> >>>     Fix fdt_fixup_mtdparts() in u-boot. This way newer U-Boot + any
> >>>     kernel should work.
> >>>
> >>> 2. To help tracking down situations like that:
> >>>     Keep the warning in ofpart.c but continue to fail.
> >>>
> >>> 3. To fix the current situation:
> >>>      Immediately revert commit (and prevent it from being backported):
> >>>      753395ea1e45 ("ARM: dts: imx7: Fix NAND controller size-cells")
> >>>      This way your own boot flow is fixed in the short term.  
> >>
> >> Here I disagree, the fix is correct and I think we shouldn't
> >> proliferate incorrect DTs which don't match the binding document.  
> > 
> > I agree we should not proliferate incorrect DTs, so let's use a modern
> > description then  
> 
> Yes please !
> 
> > , with a controller and a child node which defines the
> > chip.  
> 
> But what if there is no chip connected to the controller node ?
> 
> If I understand the proposal here right (please correct me if I'm wrong), then:

Good idea to summarize.

> 
> 1) This is the original, old, wrong binding:
> &gpmi {
>    #size-cells = <1>;
>    ...
>    partition@N { ... };
> };

Yes.

> 
> 
> 2) This is the newer, but still wrong binding:
> &gpmi {
>    #size-cells = <0>;
>    ...
>    partitions {
>      partition@N { ... };
>    };
> };

Well, this is wrong description, but it would work (for compat reasons,
even though I don't think this is considered valid DT by the schemas).

> 
> 3) This is the newest binding, what we want:
> &gpmi {
>    #size-cells = <0>;
>    ...
>    nand-chip {
>      partitions {
>        partition@N { ... };
>      };
>    };
> };

Yes

> 
> But if there is no physical nand chip connected to the controller, would we end up with empty nand-chip node in DT, like this?
> &gpmi {
>    #size-cells = <X>;
>    ...
>    nand-chip { /* empty */ };
> };

Is this really a concern? If there is no NAND chip, the controller
should be disabled, no? I guess technically you could even use the
status property in the nand-chip node...

However, it should not be empty, at the very least a reg property
should indicate on which CS it is wired, as expected there:
https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git/tree/Documentation/devicetree/bindings/mtd/nand-chip.yaml?h=mtd/next

But, as nand-chip.yaml references mtd.yaml, you can as well use
whatever is described here:
https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git/tree/Documentation/devicetree/bindings/mtd/mtd.yaml?h=mtd/next

> What would be the gpmi controller size cells (X) in that case, still 0, right ? So how does that help solve this problem, wouldn't U-Boot still populate the partitions directly under the gpmi node or into partitions sub-node ?

The commit that was pointed in the original fix clearly stated that the
NAND chip node was targeted, not the NAND controller node. I hope this
is correctly supported in U-Boot though. So if there is a NAND chip
subnode, I suppose U-Boot would try to create the partitions that are
inside, or even in the sub "partitions" container.

> >> Rather, if a bootloader generates incorrect (new) DT entries, I
> >> believe the driver should implement a fixup and warn user about this.
> >> PC does that as well with broken ACPI tables as far as I can tell.
> >>
> >> I'm not convinced making a DT non-compliant with bindings again,  
> > 
> > I am sorry to say so, but while warnings reported by the tools
> > should be fixed, it's not because the tool does not scream at you that
> > the description is valid. We are actively working on enhancing the
> > schema so that "all" improper descriptions get warnings (see the series
> > pointed earlier), but in no way this change makes the node compliant
> > with modern bindings.
> > 
> > I'm not saying the fix is wrong, but let's be pragmatic, it currently
> > leads to boot failures.  
> 
> I fully agree that we do have a problem, and that it trickled into stable makes it even worse. Maybe I don't fully understand the thing with nand-chip proposal, see my question above, esp. the last part.
> 
> >> only to work around a problem induced by bootloader, is the right approach
> >> here.  
> > 
> > When a patch breaks a board and there is no straight fix, you revert
> > it, then you think harder. That's what I am saying. This is a temporary
> > solution.  
> 
> Isn't this patch the straight fix, at least until the bootloader can be updated to generate the nand-chip node correctly ?
> 
> >> This would be setting a dangerous example, where anyone could request a DT fix to be reverted because their random bootloader does the wrong thing and with valid DT clean up, something broke.  
> > 
> > Please, you know this is not valid DT clean up. We've been decoupling
> > controller and chip description since 2016. What I am proposing is a
> > valid DT cleanup, not to the latest standard, but way closer than the
> > current solution.  
> 
> I think I really need one more explanation of the nand-chip part above.

I hope things are clearer now.

Thanks,
Miquèl

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