New patch to review for coreboot: i2c/tpm: Ignore 0xFF bytes for status and burstCount

[Patrick Georgi]

Patrick Georgi ([e-mail]) just uploaded a new patch set to gerrit, which you can find at https://review.coreboot.org/18006

-gerrit

commit 113317eafa9d8a42292b46623ef9285707503fe8
Author: Julius Werner 
Date:   Wed Nov 30 17:46:17 2016 -0800

i2c/tpm: Ignore 0xFF bytes for status and burstCount

We've found that the SLB9645 TPM sometimes seems to randomly start
    returning 0xFF bytes for all requests. The exact cause is yet unknown,
    but we should try to write our TIS code such that it avoids bad
    interactions with this kind of response (e.g. any wait_for_status()
    immediately succeeds because all "status bits" are set in the response).
    At least for status and burstCount readings we can say for sure that the
    value is nonsensical and we're already reading those in a loop until we
    get valid results anyway, so let's add code to explicitly discount 0xFF
    bytes.

BRANCH=oak
    BUG=chrome-os-partner:55764
    TEST=None

Change-Id: I934d42c36d6847a22a185795cea49d282fa113d9
    Signed-off-by: Julius Werner 
    Reviewed-on: https://chromium-review.googlesource.com/420470
    Reviewed-by: Nicolas Boichat 
---
 src/drivers/i2c/tpm/tpm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/drivers/i2c/tpm/tpm.c b/src/drivers/i2c/tpm/tpm.c
index 55c25fe..972b17d 100644
--- a/src/drivers/i2c/tpm/tpm.c
+++ b/src/drivers/i2c/tpm/tpm.c
@@ -291,6 +291,8 @@ static uint8_t tpm_tis_i2c_status(struct tpm_chip *chip)
    uint8_t buf;
    if (iic_tpm_read(TPM_STS(chip->vendor.locality), &buf, 1) < 0)
    	return 0;
+	else if (buf == 0xff)	/* Some TPMs sometimes randomly return 0xff. */
+		return 0;
    else
    	return buf;
 }
@@ -316,7 +318,7 @@ static ssize_t get_burstcount(struct tpm_chip *chip)
    	else
    		burstcnt = (buf[2] << 16) + (buf[1] << 8) + buf[0];

-		if (burstcnt)
+		if (burstcnt && burstcnt != 0xffffff)
    		return burstcnt;
    	mdelay(TPM_TIMEOUT);
    	timeout--;