Re: [PATCH] tpm_tis_spi: Don't send anything during flow control

[Doug Anderson <dianders@chromium.org>]

Hi,

On Sun, May 31, 2020 at 6:47 PM Jarkko Sakkinen
<jarkko.sakkinen@linux.intel.com> wrote:
>
> On Thu, May 28, 2020 at 03:19:30PM -0700, Douglas Anderson wrote:
> > During flow control we are just reading from the TPM, yet our spi_xfer
> > has the tx_buf and rx_buf both non-NULL which means we're requesting a
> > full duplex transfer.
> >
> > SPI is always somewhat of a full duplex protocol anyway and in theory
> > the other side shouldn't really be looking at what we're sending it
> > during flow control, but it's still a bit ugly to be sending some
> > "random" data when we shouldn't.
> >
> > The default tpm_tis_spi_flow_control() tries to address this by
> > setting 'phy->iobuf[0] = 0'.  This partially avoids the problem of
> > sending "random" data, but since our tx_buf and rx_buf both point to
> > the same place I believe there is the potential of us sending the
> > TPM's previous byte back to it if we hit the retry loop.
> >
> > Another flow control implementation, cr50_spi_flow_control(), doesn't
> > address this at all.
> >
> > Let's clean this up and just make the tx_buf NULL before we call
> > flow_control().  Not only does this ensure that we're not sending any
> > "random" bytes but it also possibly could make the SPI controller
> > behave in a slightly more optimal way.
> >
> > NOTE: no actual observed problems are fixed by this patch--it's was
> > just made based on code inspection.
> >
> > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > ---
> >
> >  drivers/char/tpm/tpm_tis_spi_main.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/char/tpm/tpm_tis_spi_main.c b/drivers/char/tpm/tpm_tis_spi_main.c
> > index d96755935529..8d2c581a93c6 100644
> > --- a/drivers/char/tpm/tpm_tis_spi_main.c
> > +++ b/drivers/char/tpm/tpm_tis_spi_main.c
> > @@ -53,8 +53,6 @@ static int tpm_tis_spi_flow_control(struct tpm_tis_spi_phy *phy,
> >
> >       if ((phy->iobuf[3] & 0x01) == 0) {
> >               // handle SPI wait states
> > -             phy->iobuf[0] = 0;
> > -
>
> Why this should be removed?

As far as I can tell the only purpose of that was to make sure we were
sending 0.  Specifically "tx_buf" "rx_buf" both point to "phy->iobuf"
so setting the first byte to 0 here made sure that we weren't sending
out "random" data but were instead sending a 0.  After my change
"tx_buf" is NULL so we don't need to do this--the controller should
take charge of sending nothing on the lines (AKA sending a zero).

Does that answer your question, or were you worried about us needing
to init iobuf[0] to 0 in some other case?

-Doug