[PATCH v2 2/2] tpm: reduce polling time to usecs for even finer granularity

[Nayna Jain]

The TPM burstcount and status commands are supposed to return very
quickly [1][2]. This patch further reduces the TPM poll sleep time to usecs
in get_burstcount() and wait_for_tpm_stat() by calling usleep_range()
directly.

After this change, performance on a TPM 1.2 with an 8 byte burstcount for
1000 extends improved from ~10.7 sec to ~7 sec.

[1] From TCG Specification "TCG PC Client Specific TPM Interface
Specification (TIS), Family 1.2":

"NOTE : It takes roughly 330 ns per byte transfer on LPC. 256 bytes would
take 84 us, which is a long time to stall the CPU. Chipsets may not be
designed to post this much data to LPC; therefore, the CPU itself is
stalled for much of this time. Sending 1 kB would take 350 Îs. Therefore,
even if the TPM_STS_x.burstCount field is a high value, software SHOULD
be interruptible during this period."

[2] From TCG Specification 2.0, "TCG PC Client Platform TPM Profile
(PTP) Specification":

"It takes roughly 330 ns per byte transfer on LPC. 256 bytes would take
84 us. Chipsets may not be designed to post this much data to LPC;
therefore, the CPU itself is stalled for much of this time. Sending 1 kB
would take 350 us. Therefore, even if the TPM_STS_x.burstCount field is a
high value, software should be interruptible during this period. For SPI,
assuming 20MHz clock and 64-byte transfers, it would take about 120 usec
to move 256B of data. Sending 1kB would take about 500 usec. If the
transactions are done using 4 bytes at a time, then it would take about
1 msec. to transfer 1kB of data."

Signed-off-by: Nayna Jain <nayna@xxxxxxxxxxxxxxxxxx>
---
 drivers/char/tpm/tpm.h          | 4 +++-
 drivers/char/tpm/tpm_tis_core.c | 5 +++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
index 7e797377e1eb..f0e4d290c347 100644
--- a/drivers/char/tpm/tpm.h
+++ b/drivers/char/tpm/tpm.h
@@ -54,7 +54,9 @@ enum tpm_timeout {
 	TPM_TIMEOUT = 5,	/* msecs */
 	TPM_TIMEOUT_RETRY = 100, /* msecs */
 	TPM_TIMEOUT_RANGE_US = 300,	/* usecs */
-	TPM_TIMEOUT_POLL = 1	/* msecs */
+	TPM_TIMEOUT_POLL = 1,	/* msecs */
+	TPM_TIMEOUT_USECS_MIN = 100,      /* usecs */
+	TPM_TIMEOUT_USECS_MAX = 500      /* usecs */
 };
 
 /* TPM addresses */
diff --git a/drivers/char/tpm/tpm_tis_core.c b/drivers/char/tpm/tpm_tis_core.c
index 021e6b68f2db..5bba5c662423 100644
--- a/drivers/char/tpm/tpm_tis_core.c
+++ b/drivers/char/tpm/tpm_tis_core.c
@@ -84,7 +84,8 @@ static int wait_for_tpm_stat(struct tpm_chip *chip, u8 mask,
 		}
 	} else {
 		do {
-			tpm_msleep(TPM_TIMEOUT_POLL);
+			usleep_range(TPM_TIMEOUT_USECS_MIN,
+					TPM_TIMEOUT_USECS_MAX);
 			status = chip->ops->status(chip);
 			if ((status & mask) == mask)
 				return 0;
@@ -226,7 +227,7 @@ static int get_burstcount(struct tpm_chip *chip)
 		burstcnt = (value >> 8) & 0xFFFF;
 		if (burstcnt)
 			return burstcnt;
-		tpm_msleep(TPM_TIMEOUT_POLL);
+		usleep_range(TPM_TIMEOUT_USECS_MIN, TPM_TIMEOUT_USECS_MAX);
 	} while (time_before(jiffies, stop));
 	return -EBUSY;
 }
-- 
2.13.3