[PATCH v2 08/10] tpm: Add the RNG child device

[Simon Glass]

Hi Sughosh,

On Thu, 3 Mar 2022 at 05:11, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
>
> hi Simon,
>
> On Thu, 3 Mar 2022 at 09:18, Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Sughosh,
> >
> > On Tue, 1 Mar 2022 at 21:53, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> > >
> > > hi Simon,
> > >
> > > On Tue, 1 Mar 2022 at 20:29, Simon Glass <sjg at chromium.org> wrote:
> > > >
> > > > Hi Sughosh,
> > > >
> > > > On Mon, 28 Feb 2022 at 05:07, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> > > > >
> > > > > The TPM device comes with the random number generator(RNG)
> > > > > functionality which is built into the TPM device. Add logic to add the
> > > > > RNG child device in the TPM uclass post probe callback.
> > > > >
> > > > > The RNG device can then be used to pass a set of random bytes to the
> > > > > linux kernel, need for address space randomisation through the
> > > > > EFI_RNG_PROTOCOL interface.
> > > > >
> > > > > Signed-off-by: Sughosh Ganu <sughosh.ganu at linaro.org>
> > > > > ---
> > > > >
> > > > > Changes since V1: None
> > > > >
> > > > >  drivers/tpm/tpm-uclass.c | 58 +++++++++++++++++++++++++++++++++++++---
> > > > >  1 file changed, 54 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/drivers/tpm/tpm-uclass.c b/drivers/tpm/tpm-uclass.c
> > > > > index 8619da89d8..383cc7bc48 100644
> > > > > --- a/drivers/tpm/tpm-uclass.c
> > > > > +++ b/drivers/tpm/tpm-uclass.c
> > > > > @@ -16,6 +16,11 @@
> > > > >  #include <tpm-v2.h>
> > > > >  #include "tpm_internal.h"
> > > > >
> > > > > +#include <dm/lists.h>
> > > > > +
> > > > > +#define TPM_RNG1_DRV_NAME      "tpm1-rng"
> > > > > +#define TPM_RNG2_DRV_NAME      "tpm2-rng"
> > > > > +
> > > > >  bool is_tpm1(struct udevice *dev)
> > > > >  {
> > > > >         return IS_ENABLED(CONFIG_TPM_V1) && tpm_get_version(dev) == TPM_V1;
> > > > > @@ -147,12 +152,57 @@ int tpm_xfer(struct udevice *dev, const uint8_t *sendbuf, size_t send_size,
> > > > >         return 0;
> > > > >  }
> > > > >
> > > > > +#if IS_ENABLED(CONFIG_TPM)
> > > >
> > > > This should be in the Makefile so that we only build this file if TPM
> > > > is enabled.
> > >
> > > The Makefile allows for the tpm uclass driver to be built for SPL and
> > > TPL stages as well. The addition of the RNG device is to be done only
> > > in the u-boot proper stage, since we enable RNG support only in u-boot
> > > proper. Thanks.
> >
> > Well in that case, create a new SPL_TPM_RAND or similar to control
> > enabling it in SPL. It should be explicit.
>
> I think it is easier to just protect the child addition functions
> under CONFIG_TPM rather than create SPL_RNG and TPL_RNG symbols. We
> don't have any requirement for generating random numbers in the SPL
> and TPL stages. I feel that creating new symbols just for the sake of
> not putting a check for CONFIG_TPM is a bit of an overkill, especially
> since we do not have any requirement for RNG devices in the SPL/TPL
> stages.

What does checking for CONFIG_TPM have to do with SPL and TPL? If that
option is enabled, the feature will be active in SPL and TPL too.

Also I see another problem, on further examination. You cannot start
up the TPM in the pre_probe() function. That needs to be done under
board control. E.g. for coral it happens in the TPL (or soon VPL)
phase so cannot be done again in U-Boot proper.

So perhaps we need to remember the state of the TPM (using SPL handoff
perhaps). Also you probably need to move the startup stuff to the RNG
itself.

Perhaps we could add a new function to handle this, which can be
called from your rand driver.

int tpm_ensure_started(struct udevice *dev, enum tpm_startup_type mode)

Regards,
Simon