Messages in this thread | | | Subject | Re: [PATCH] tpm_tis_spi: Don't send anything during flow control | From | Paul Menzel <> | Date | Fri, 29 May 2020 10:33:28 +0200 |
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Dear Douglas,
Thank you for the patch.
Am 29.05.20 um 00:19 schrieb Douglas Anderson: > 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.
s/it's was/it was/
Were you able to test this? Maybe in the “Chromebook QA arsenal”? Are you already running it in production on Google Chrome OS devices?
> 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; > - > for (i = 0; i < TPM_RETRY; i++) { > spi_xfer->len = 1; > spi_message_init(&m); > @@ -104,6 +102,8 @@ int tpm_tis_spi_transfer(struct tpm_tis_data *data, u32 addr, u16 len, > if (ret < 0) > goto exit; > > + /* Flow control transfers are receive only */ > + spi_xfer.tx_buf = NULL; > ret = phy->flow_control(phy, &spi_xfer); > if (ret < 0) > goto exit; > @@ -113,9 +113,8 @@ int tpm_tis_spi_transfer(struct tpm_tis_data *data, u32 addr, u16 len, > spi_xfer.delay.value = 5; > spi_xfer.delay.unit = SPI_DELAY_UNIT_USECS; > > - if (in) { > - spi_xfer.tx_buf = NULL; > - } else if (out) { > + if (out) { > + spi_xfer.tx_buf = phy->iobuf; > spi_xfer.rx_buf = NULL; > memcpy(phy->iobuf, out, transfer_len); > out += transfer_len;
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Kind regards,
Paul
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