tpm_tis_spi: Don't send anything during flow control

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>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

diff --git a/drivers/char/tpm/tpm_tis_spi_main.c b/drivers/char/tpm/tpm_tis_spi_main.c
index d967559..8d2c581 100644 (file)
--- 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;