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

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

miquel.raynal@bootlin.com wrote on Fri, 2 Dec 2022 16:49:04 +0100:

> 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

Perhaps I should also mention that #size-cells expected to be 0 has
nothing to do with the "partitions" container (otherwise #address-cells
would be 0 as well). This value is however asking for an address-only
reg property describing which NAND chip should be addressed and how,
basically the NAND controller CS because you can wire your NAND to
any CS.

> > 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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