[PATCH v6 3/7] tpm: Add the RNG child device

[Simon Glass]

Hi Rob,

On Tue, 12 Jul 2022 at 08:11, Rob Herring <robh at kernel.org> wrote:
>
> On Tue, Jul 12, 2022 at 5:04 AM Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Ilias,
> >
> > On Fri, 8 Jul 2022 at 02:24, Ilias Apalodimas
> > <ilias.apalodimas at linaro.org> wrote:
> > >
> > > Hi Simon,
> > >
> > > [...]
> > >
> > > > > +
> > > > >  UCLASS_DRIVER(tpm) = {
> > > > > -       .id             = UCLASS_TPM,
> > > > > -       .name           = "tpm",
> > > > > -       .flags          = DM_UC_FLAG_SEQ_ALIAS,
> > > > > +       .id                     = UCLASS_TPM,
> > > > > +       .name                   = "tpm",
> > > > > +       .flags                  = DM_UC_FLAG_SEQ_ALIAS,
> > > > >  #if CONFIG_IS_ENABLED(OF_REAL)
> > > > > -       .post_bind      = dm_scan_fdt_dev,
> > > > > +       .post_bind              = dm_scan_fdt_dev,
> > > > >  #endif
> > > > > +       .post_probe             = tpm_uclass_post_probe,
> > > > >         .per_device_auto        = sizeof(struct tpm_chip_priv),
> > > > >  };
> > > > > --
> > > > > 2.25.1
> > > > >
> > > >
> > > > The driver needs a compatible string so it can be in the device tree.
> > >
> > > Why?  I've tried to hint this on the previous iteration of the patch.
> > > The RNG here is not a *device*.  The TPM is the device and you are
> > > guaranteed to have an RNG.  The way to get a random number is send a
> > > special command to the TPM. So all that we should do here is leverage
> > > the fact that the TPM is already in the device tree.
> > >
> > > And fwiw we should stick to try to stick to what the DT spec defines
> > > as much as possible.  I personally don't see this as a special usecase
> > > were deviating from the spec is justified.
> >
> > This is not a deviation from a spec. What spec? Also, I don't want to
> > get into another discussion about what a device is. We can disagree on
> > that if you like.
> >
> > One reason is that U-Boot generally requires compatible strings, e.g.
> > with of-platdata. But also we can refer to the rand device from
> > elsewhere in the tree. I know that Linux does lots of ad-hoc device
> > creation and doesn't really have the concept of a uclass consistently
> > applied, but this is U-Boot.
>
> You are letting client/OS details define your binding. Doing so
> doesn't result in OS agnostic bindings. Sure, it would be nice if DT
> nodes and drivers were always a nice clean 1:1 ratio, but h/w is messy
> sometimes and DT is not an abstraction layer. The general guidance on
> whether there are child nodes for sub-blocks is do they have their own
> resources in DT or are the sub-blocks reusable on multiple devices.
> Neither of those are the case here.
>
> Besides, we already have TPM device bindings defined. Requiring
> binding changes when adding a new client/OS feature is not good
> practice either.

I'm not sure what to do with this, but in general the practice of
implied subnodes is not friendly to U-Boot. Yet it seems like a common
feature of the bindings at present, for example GPIOs.

The device tree is how U-Boot determines which random-number generator
to use for a particular function. For example, for VBE, if we need to
generate some random numbers we'd like to specify which device creates
them. It can't just be 'use the TPM if you find one'. I'm not sure how
that works in Linux?

Regards,
SImon