[PATCH v5 4/9] tpm: Add the RNG child device

[Simon Glass]

Hi Sughosh,

On Mon, 14 Mar 2022 at 05:43, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
>
> hi Simon,
>
> On Mon, 14 Mar 2022 at 03:53, Simon Glass <sjg at chromium.org> wrote:
> >
> > Hi Sughosh,
> >
> > On Sun, 13 Mar 2022 at 08:48, 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 V4:
> > >
> > > * Put a check for CONFIG_TPM_RNG for binding the RNG device with it's
> > >   driver in the post_probe callback instead of putting
> > >   CONFIG_{SPL,TPL}_BUILD guards
> > >
> > >  drivers/tpm/tpm-uclass.c | 29 +++++++++++++++++++++++++----
> > >  1 file changed, 25 insertions(+), 4 deletions(-)
> > >
> >
> > This looks a lot better, please see below.
> >
> > > diff --git a/drivers/tpm/tpm-uclass.c b/drivers/tpm/tpm-uclass.c
> > > index f67fe1019b..e1c61d26f0 100644
> > > --- a/drivers/tpm/tpm-uclass.c
> > > +++ b/drivers/tpm/tpm-uclass.c
> > > @@ -11,10 +11,15 @@
> > >  #include <log.h>
> > >  #include <linux/delay.h>
> > >  #include <linux/unaligned/be_byteshift.h>
> > > +#include <tpm_api.h>
> > >  #include <tpm-v1.h>
> > >  #include <tpm-v2.h>
> > >  #include "tpm_internal.h"
> > >
> > > +#include <dm/lists.h>
> > > +
> > > +#define TPM_RNG_DRV_NAME       "tpm-rng"
> > > +
> > >  int tpm_open(struct udevice *dev)
> > >  {
> > >         struct tpm_ops *ops = tpm_get_ops(dev);
> > > @@ -136,12 +141,28 @@ int tpm_xfer(struct udevice *dev, const uint8_t *sendbuf, size_t send_size,
> > >         return 0;
> > >  }
> > >
> > > +static int tpm_uclass_post_probe(struct udevice *dev)
> > > +{
> > > +       int ret;
> > > +       const char *drv = TPM_RNG_DRV_NAME;
> > > +       struct udevice *child;
> > > +
> > > +       if (CONFIG_IS_ENABLED(TPM_RNG)) {
> > > +               ret = device_bind_driver(dev, drv, "tpm-rng0", &child);
> > > +               if (ret)
> > > +                       return log_msg_ret("bind", ret);
> > > +       }
> >
> > This really should be in the device tree so what you are doing here is
> > quite strange.
>
> Like I had mentioned in my earlier emails, the TPM device has a
> builtin RNG functionality, which is non-optional. So I don't
> understand why we need to use a device tree subnode here. Whether the
> device is being bound to the parent is being controlled by the TPM_RNG
> config that you asked me to put in my previous version, which I am
> doing.

See how PMICs work, for example. We have GPIOs, regulators and
suchlike in the PMIC and we add subnodes for them in the DT. It is
just how it is done.

Driver model is designed to automatically bind devices based on the
device tree. There are cases where it is necessary to manually bind
things, but we mustn't prevent people from doing it 'properly'.

Finally, I know you keep saying that random numbers are only needed in
U-Boot proper, but if I want a random number in SPL, it may not work,
since device_bind() is often not available, for code-size reasons.

So that is why I say that what you are doing is quite strange. Perhaps
you are coming from a different project, which does things
differently.

>
>  If you want to manually bind it, please call
> > device_find_first_child_by_uclass() first to make sure it isn't
> > already there.
>
> Okay
>
> >
> > Also you should bind it in the bind() method, not in probe().
>
> Okay
>
> >
> > This is the code used for the same thing in the bootstd series:
> >
> > struct udevice *bdev;
> > int ret;
> >
> > ret = device_find_first_child_by_uclass(parent, UCLASS_BOOTDEV, &bdev);
> > if (ret) {
> >    if (ret != -ENODEV) {
> >    log_debug("Cannot access bootdev device\n");
> >    return ret;
> >    }
> >
> >    ret = bootdev_bind(parent, drv_name, "bootdev", &bdev);
> >    if (ret) {
> >       log_debug("Cannot create bootdev device\n");
> >       return ret;
> >    }
> > }
> >
> >
> > > +
> > > +       return 0;
> > > +}
> > > +
> > >  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,
> >
> > Should be post_bind.
>
> Okay

[..]

Regards,
Simon