linux-kernel - [PATCH 2/6] RTC/i.MX/DryIce: add some background info about the states the machine can be in

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Message-Id: <1430143192-20667-3-git-send-email-jbe@pengutronix.de>
Date:	Mon, 27 Apr 2015 15:59:48 +0200
From:	Juergen Borleis <jbe@...gutronix.de>
To:	linux-kernel@...r.kernel.org
Cc:	rtc-linux@...glegroups.com, kernel@...gutronix.de,
	Alessandro Zummo <a.zummo@...ertech.it>,
	linux-arm-kernel@...ts.infradead.org,
	Robert Schwebel <rsc@...gutronix.de>
Subject: [PATCH 2/6] RTC/i.MX/DryIce: add some background info about the states the machine can be in

Signed-off-by: Juergen Borleis <jbe@...gutronix.de>
Signed-off-by: Robert Schwebel <rsc@...gutronix.de>
[rsc: got NDA clearance from Freescale]
---
 drivers/rtc/rtc-imxdi.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/drivers/rtc/rtc-imxdi.c b/drivers/rtc/rtc-imxdi.c
index 0c2a064..6db8d1c 100644
--- a/drivers/rtc/rtc-imxdi.c
+++ b/drivers/rtc/rtc-imxdi.c
@@ -129,6 +129,49 @@ struct imxdi_dev {
 	struct work_struct work;
 };
 
+/* Some background:
+ *
+ * The DryIce unit is a complex security/tamper monitor device. To be able do
+ * its job in a useful manner it runs a bigger statemachine to bring it into
+ * security/tamper failure state and once again to bring it out of this state.
+ *
+ * This unit can be in one of three states:
+ *
+ * - "NON-VALID STATE"
+ *   always after the battery power was removed
+ * - "FAILURE STATE"
+ *   if one of the enabled security events has happened
+ * - "VALID STATE"
+ *   if the unit works as expected
+ *
+ * Everything stops when the unit enters the failure state including the RTC
+ * counter (to be able to detect the time the security event happened).
+ *
+ * The following events (when enabled) let the DryIce unit enter the failure
+ * state:
+ *
+ * - wire-mesh-tamper detect
+ * - external tamper B detect
+ * - external tamper A detect
+ * - temperature tamper detect
+ * - clock tamper detect
+ * - voltage tamper detect
+ * - RTC counter overflow
+ * - monotonic counter overflow
+ * - external boot
+ *
+ * If we find the DryIce unit in "FAILURE STATE" and the TDCHL cleared, we
+ * can only detect this state. In this case the unit is completely locked and
+ * must force a second "SYSTEM POR" to bring the DryIce into the
+ * "NON-VALID STATE" + "FAILURE STATE" where a recovery is possible.
+ * If the TDCHL is set in the "FAILURE STATE" we are out of luck. In this case
+ * a battery power cycle is required.
+ *
+ * In the "NON-VALID STATE" + "FAILURE STATE" we can clear the "FAILURE STATE"
+ * and recover the DryIce unit. By clearing the "NON-VALID STATE" as the last
+ * task, we bring back this unit into life.
+ */
+
 /*
  * enable a dryice interrupt
  */
-- 
2.1.4

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