Change in coreboot[master]: spi/tpm.c do not waste time on wake pulses unless necessary

[Vadim Bendebury (Code Review)]

Vadim Bendebury has uploaded this change for review. ( https://review.coreboot.org/22321

Change subject: spi/tpm.c do not waste time on wake pulses unless necessary
......................................................................

spi/tpm.c do not waste time on wake pulses unless necessary

The Cr50 secure chip implementation is guaranteed not to fall asleep
for 1 second after any SPI slave activity.

Let's not waste time on the wake up ping when it is not necessary.

BRANCH=cr50
BUG=b:68012381
TEST=using a protocol analyzer verified that the wake pulses are
     generated only when the new Coreboot stage or Depthcharge start,
     not on every SPI slave transaction.

Change-Id: Id8def1470ba3eab533075b9e7180f8a58e0b00b6
Signed-off-by: Vadim Bendebury 
---
M src/drivers/spi/tpm/tpm.c
1 file changed, 28 insertions(+), 5 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/21/22321/1

diff --git a/src/drivers/spi/tpm/tpm.c b/src/drivers/spi/tpm/tpm.c
index 58bf842..c47b807 100644
--- a/src/drivers/spi/tpm/tpm.c
+++ b/src/drivers/spi/tpm/tpm.c
@@ -109,6 +109,9 @@
    struct stopwatch sw;
    static int tpm_sync_needed CAR_GLOBAL;
    struct spi_slave *spi_slave = car_get_var_ptr(&g_spi_slave);
+	int wakeup_needed = 1;
+	static long prev_us CAR_GLOBAL;
+	struct mono_time mt;

/* Wait for tpm to finish previous transaction if needed */
    if (car_get_var(tpm_sync_needed))
@@ -116,11 +119,31 @@
    else
    	car_set_var(tpm_sync_needed, 1);

-	/* Try to wake cr50 if it is asleep. */
-	spi_claim_bus(spi_slave);
-	udelay(1);
-	spi_release_bus(spi_slave);
-	udelay(100);
+	if (IS_ENABLED(CONFIG_HAVE_MONOTONIC_TIMER)) {
+		timer_monotonic_get(&mt);
+
+		if (car_get_var(prev_us))
+			/*
+			 * If there is a timestamp, check if SPI slave timeout
+			 * has expired.
+			 *
+			 * The Cr50 on H1 does not go to sleep for 1 second
+			 * after any SPI slave activity, let's be conservative
+			 * and limit the window to 900 ms.
+			 */
+			if (mt.microseconds <
+			    (car_get_var(prev_us) + 900 * 1000))
+				wakeup_needed = 0;
+		car_set_var(prev_us, mt.microseconds);
+	}
+
+	if (wakeup_needed) {
+		/* Just in case Cr50 is asleep. */
+		spi_claim_bus(spi_slave);
+		udelay(1);
+		spi_release_bus(spi_slave);
+		udelay(100);
+	}

/*
     * The first byte of the frame header encodes the transaction type