Hi Rob,
On Mon, Jan 30, 2023 at 8:36 PM Rob Herring <robh@xxxxxxxxxx> wrote:
> On Wed, Jan 25, 2023 at 10:42:13AM +0000, Robin Murphy wrote:
> > On 2023-01-25 08:54, Geert Uytterhoeven wrote:
> > > On Wed, Jan 25, 2023 at 1:49 AM Yoshihiro Shimoda
> > > <yoshihiro.shimoda.uh@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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@xxxxxxxxxxx>
> > >
> > > > > > ---
> > > > > > 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.
But it does not have to be two-dimensional.
The second dimension was added in commit 39bd2b6a3783b899 ("dt-bindings
Improve phandle-array schemas"), but is not needed.
Can this be simplified again?
The driver doesn't care, it just checks for the presence of the
property, i.e. treats it as a boolean flag.
> A 4th option is a new property.
If all else fails, a new boolean flag would work...
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
