[RFC PATCH] tpm: msleep() delays - replace with usleep_range() in i2c nuvoton driver

[Mimi Zohar <zohar-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

Commit 500462a9de65 "timers: Switch to a non-cascading wheel" replaced
the 'classic' timer wheel, which aimed for near 'exact' expiry of the
timers.  Their analysis was that the vast majority of timeout timers
are used as safeguards, not as real timers, and are cancelled or
rearmed before expiration.  The only exception noted to this were
networking timers with a small expiry time.

Not included in the analysis was the TPM polling timer, which resulted
in a longer normal delay and, every so often, a very long delay.  The
non-cascading wheel delay is based on CONFIG_HZ.  For a description of
the different rings and their delays, refer to the comments in
kernel/time/timer.c.

Below are the delays given for rings 0 - 2, which explains the longer
"normal" delays and the very, long delays as seen on systems with
CONFIG_HZ 250.

* HZ 1000 steps
 * Level Offset  Granularity            Range
 *  0      0         1 ms                0 ms - 63 ms
 *  1     64         8 ms               64 ms - 511 ms
 *  2    128        64 ms              512 ms - 4095 ms (512ms - ~4s)

* HZ  250
 * Level Offset  Granularity            Range
 *  0      0         4 ms                0 ms - 255 ms
 *  1     64        32 ms              256 ms - 2047 ms (256ms - ~2s)
 *  2    128       256 ms             2048 ms - 16383 ms (~2s - ~16s)

Below is a comparison of extending the TPM with 1000 measurements,
using msleep() vs. usleep_delay() when configured for 1000 hz vs. 250
hz, before and after commit 500462a9de65.

		linux-4.7 | msleep()	usleep_range()
1000 hz:	0m44.628s | 1m34.497s	29.243s
250 hz:		1m28.510s | 4m49.269s	32.386s

		linux-4.7 	| min-max (msleep)  min-max (usleep_range)
1000 hz:	0:017 - 2:760s	| 0:015 - 3:967s    0:014 - 0:418s
250 hz:		0:028 - 1:954s	| 0:040 - 4:096s    0:016 - 0:816s

This patch replaces the msleep() with usleep_range() calls in the
i2c nuvoton driver with a consistent max range value.

Signed-of-by: Mimi Zohar <zohar-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
Reviewed-by: Nayna Jain <nayna-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
---
 drivers/char/tpm/tpm_i2c_nuvoton.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/drivers/char/tpm/tpm_i2c_nuvoton.c b/drivers/char/tpm/tpm_i2c_nuvoton.c
index e3a9155ee671..da2508a6bc0c 100644
--- a/drivers/char/tpm/tpm_i2c_nuvoton.c
+++ b/drivers/char/tpm/tpm_i2c_nuvoton.c
@@ -49,9 +49,9 @@
  */
 #define TPM_I2C_MAX_BUF_SIZE           32
 #define TPM_I2C_RETRY_COUNT            32
-#define TPM_I2C_BUS_DELAY              1       /* msec */
-#define TPM_I2C_RETRY_DELAY_SHORT      2       /* msec */
-#define TPM_I2C_RETRY_DELAY_LONG       10      /* msec */
+#define TPM_I2C_BUS_DELAY              1000      /* usec */
+#define TPM_I2C_RETRY_DELAY_SHORT      2 * 1000  /* usec */
+#define TPM_I2C_RETRY_DELAY_LONG       10 * 1000 /* usec */
 
 #define OF_IS_TPM2 ((void *)1)
 #define I2C_IS_TPM2 1
@@ -123,7 +123,7 @@ static s32 i2c_nuvoton_write_status(struct i2c_client *client, u8 data)
 	/* this causes the current command to be aborted */
 	for (i = 0, status = -1; i < TPM_I2C_RETRY_COUNT && status < 0; i++) {
 		status = i2c_nuvoton_write_buf(client, TPM_STS, 1, &data);
-		msleep(TPM_I2C_BUS_DELAY);
+		usleep_range(TPM_I2C_BUS_DELAY, TPM_I2C_BUS_DELAY + 300);
 	}
 	return status;
 }
@@ -160,7 +160,7 @@ static int i2c_nuvoton_get_burstcount(struct i2c_client *client,
 			burst_count = min_t(u8, TPM_I2C_MAX_BUF_SIZE, data);
 			break;
 		}
-		msleep(TPM_I2C_BUS_DELAY);
+		usleep_range(TPM_I2C_BUS_DELAY, TPM_I2C_BUS_DELAY + 300);
 	} while (time_before(jiffies, stop));
 
 	return burst_count;
@@ -203,13 +203,15 @@ static int i2c_nuvoton_wait_for_stat(struct tpm_chip *chip, u8 mask, u8 value,
 			return 0;
 
 		/* use polling to wait for the event */
-		ten_msec = jiffies + msecs_to_jiffies(TPM_I2C_RETRY_DELAY_LONG);
+		ten_msec = jiffies + usecs_to_jiffies(TPM_I2C_RETRY_DELAY_LONG);
 		stop = jiffies + timeout;
 		do {
 			if (time_before(jiffies, ten_msec))
-				msleep(TPM_I2C_RETRY_DELAY_SHORT);
+				usleep_range(TPM_I2C_RETRY_DELAY_SHORT,
+					     TPM_I2C_RETRY_DELAY_SHORT + 300);
 			else
-				msleep(TPM_I2C_RETRY_DELAY_LONG);
+				usleep_range(TPM_I2C_RETRY_DELAY_LONG,
+					     TPM_I2C_RETRY_DELAY_LONG + 300);
 			status_valid = i2c_nuvoton_check_status(chip, mask,
 								value);
 			if (status_valid)
-- 
2.9.3


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