linux-kernel - [PATCH] mfd: cros ec: spi: Increase wait time to 200ms

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Message-Id: <20170301115832.21181-1-enric.balletbo@collabora.com>
Date:   Wed,  1 Mar 2017 12:58:32 +0100
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     linux-kernel@...r.kernel.org, Doug Anderson <dianders@...omium.org>
Subject: [PATCH] mfd: cros ec: spi: Increase wait time to 200ms

From: Doug Anderson <dianders@...omium.org>

This is a sucky change to bump up the time we'll wait for the EC.  Why
is it sucky?  If 200ms for a transfer is a common thing it will have a
massively bad impact on keyboard responsiveness.

It still seems like a good idea to do this, though, because we have a
gas gauge that claims that in an extreme case it could stretch the i2c
clock for 144ms.  It's not a common case so it shouldn't affect
responsiveness, but it can happen.  It's much better to have a single
slow keyboard response than to start returning errors when we don't
have to.

In newer EC designs we should probably implement a virtual battery to
respond to the kernel to insulate the kernel from these types of
issues.

Signed-off-by: Doug Anderson <dianders@...omium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
---
 This fixes a random BUG report in cros-ec-spi in my rk3399 gru device.

 drivers/mfd/cros_ec_spi.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/mfd/cros_ec_spi.c b/drivers/mfd/cros_ec_spi.c
index a518832..c971407 100644
--- a/drivers/mfd/cros_ec_spi.c
+++ b/drivers/mfd/cros_ec_spi.c
@@ -45,8 +45,11 @@
  * on the other end and need to transfer ~256 bytes, then we need:
  *  10 us/bit * ~10 bits/byte * ~256 bytes = ~25ms
  *
- * We'll wait 4 times that to handle clock stretching and other
- * paranoia.
+ * We'll wait 8 times that to handle clock stretching and other
+ * paranoia.  Note that some battery gas gauge ICs claim to have a
+ * clock stretch of 144ms in rare situations.  That's incentive for
+ * not directly passing i2c through, but it's too late for that for
+ * existing hardware.
  *
  * It's pretty unlikely that we'll really see a 249 byte tunnel in
  * anything other than testing.  If this was more common we might
@@ -54,7 +57,7 @@
  * wait loop.  The 'flash write' command would be another candidate
  * for this, clocking in at 2-3ms.
  */
-#define EC_MSG_DEADLINE_MS		100
+#define EC_MSG_DEADLINE_MS		200
 
 /*
   * Time between raising the SPI chip select (for the end of a
-- 
2.9.3