On Wed, Apr 15, 2020 at 04:51:39PM -0700, James Bottomley wrote:
> On Wed, 2020-04-15 at 15:45 -0700, Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@xxxxxx>
> >
> > We've encountered a particular model of STMicroelectronics TPM that
> > transiently returns a bad value in the status register. This causes
> > the kernel to believe that the TPM is ready to receive a command when
> > it actually isn't, which in turn causes the send to time out in
> > get_burstcount(). In testing, reading the status register one extra
> > time convinces the TPM to return a valid value.
>
> Interesting, I've got a very early upgradeable nuvoton that seems to be
> behaving like this.
>
> > Signed-off-by: Omar Sandoval <osandov@xxxxxx>
> > ---
> > drivers/char/tpm/tpm_tis_core.c | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/char/tpm/tpm_tis_core.c
> > b/drivers/char/tpm/tpm_tis_core.c
> > index 27c6ca031e23..277a21027fc7 100644
> > --- a/drivers/char/tpm/tpm_tis_core.c
> > +++ b/drivers/char/tpm/tpm_tis_core.c
> > @@ -238,6 +238,18 @@ static u8 tpm_tis_status(struct tpm_chip *chip)
> > rc = tpm_tis_read8(priv, TPM_STS(priv->locality), &status);
> > if (rc < 0)
> > return 0;
> > + /*
> > + * Some STMicroelectronics TPMs have a bug where the status
> > register is
> > + * sometimes bogus (all 1s) if read immediately after the
> > access
> > + * register is written to. Bits 0, 1, and 5 are always
> > supposed to read
> > + * as 0, so this is clearly invalid. Reading the register a
> > second time
> > + * returns a valid value.
> > + */
> > + if (unlikely(status == 0xff)) {
> > + rc = tpm_tis_read8(priv, TPM_STS(priv->locality),
> > &status);
> > + if (rc < 0)
> > + return 0;
> > + }
>
> You theorize that your case is fixed by the second read, but what if it
> isn't and the second read also returns 0xff? Shouldn't we have a line
> here saying
>
> if (unlikely(status == 0xff))
> status = 0;
>
> So if we get a second 0xff we just pretend the thing isn't ready?
If it eventually settles, would it be better to poll it for a while?
Also, the commit message is ambiguous. "bad value" can be any random
bit sequence.
/Jarkko
