[PATCH] tpm_tis: work around status register bug in STMicroelectronics TPM

[Omar Sandoval <osandov@osandov.com>]

From: Omar Sandoval <osandov@fb.com>

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.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
 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;
+	}
 
 	return status;
 }
-- 
2.26.1