Re: [PATCH] dt-bindings: iommu: renesas,ipmmu-vmsa: Update descriptions for R-Car Gen4

[Rob Herring <robh@kernel.org>]

On Wed, Jan 25, 2023 at 10:42:13AM +0000, Robin Murphy wrote:
> On 2023-01-25 08:54, Geert Uytterhoeven wrote:
> > Hi Shimoda-san,
> > 
> > On Wed, Jan 25, 2023 at 1:49 AM Yoshihiro Shimoda
> > <yoshihiro.shimoda.uh@renesas.com> wrote:
> > > > From: Geert Uytterhoeven, Sent: Tuesday, January 24, 2023 11:35 PM
> > > > On Mon, Jan 23, 2023 at 2:35 AM Yoshihiro Shimoda
> > > > <yoshihiro.shimoda.uh@renesas.com> wrote:
> > > > > Since R-Car Gen4 doens't have the main IPMMU IMSSTR register, but
> > > > > each cache IPMMU has own module id. So, update descriptions of
> > > > > renesas,ipmmu-main property for R-Car Gen4.
> > > > > 
> > > > > Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> > 
> > > > > ---
> > > > >   The old R-Car S4-8 datasheet had described IPMMU IMSSTR register, but
> > > > >   the latest datasheet undocumented the register. So, update the propeties
> > > > >   description. Note that the second argument is not used on the driver.
> > > > 
> > > > DT describes hardware, not software policy.
> > > 
> > > I think so.
> > > 
> > > > >   So no behavior change.
> > > > 
> > > > So where do we get the module id numbers to use, if they are no longer
> > > > documented in the Hardware Manual?
> > > 
> > > If so, we cannot get the module id numbers. So, should we use other
> > > information which is completely fixed instead? I have some ideas:
> > > 1) Just 0 (or other fixed value) if the IMSSTR register doesn't exist.
> > > 2) Sequential numbers from register base offset.
> > >     In R-Car S4: ipmmu_rt0 is the first node from register base offset,
> > >     and ipmmu_rt1 is the second one.
> > >     So, ipmmu_rt0 is 0, ipmmu_rt1 is 1, ipmmu_ds0 is 2 and ipmmu_hc is 3.
> > > 3) Using base address upper 16-bits.
> > >     In R-Car S4: ipmmu_rt0 is 0xee480000. So, the value is 0xee48.
> > > 
> > > Perhaps, the option 1) is reasonable, I think. But, what do you think?
> > 
> > I would not make up numbers, as that would cause confusion with SoCs
> > where the numbers do match the hardware.
> > As the driver doesn't use the module id number (it already loops
> > over all domains, instead of checking IMSSTR, probably because of
> > historical (R-Car Gen2) reasons?), what about dropping it from the
> > property? I.e. add "minItems: 1", possibly only when compatible with
> > renesas,rcar-gen4-ipmmu-vmsa?
> 
> Right, if there really is no meaningful ID for this model then its binding
> should not require one.

I agree, however that makes parsing the property a pain (for both the 
schema and driver). This property is a matrix. The number of entries is 
already variable. If both dimensions are variable, we have to then look 
at the compatible to know how to parse it. I would go with option 1.

A 4th option is a new property.

Rob