[PATCH] applesmc remedy take 2

[Henrik Rydberg]

Conjectured problem: there are remnant bytes ready on the data line
which corrupts the read after a failure.

Remedy: assuming bit0 is the read valid line, try to flush it before
starting a new command.

Tests by Chris suggests reading the status is enough for the problem
to go away, which is consistent with a change in the SMC interface,
where the number of bytes to read is no longer specified, but found
out by reading until end of data.

Tested on a MacBookAir3,1, but the original problem has not been
reproduced.
---
 drivers/hwmon/applesmc.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index 98814d1..c0ff350 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -230,6 +230,7 @@ static int send_argument(const char *key)
 
 static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 {
+	u8 status, data;
 	int i;
 
 	if (send_command(cmd) || send_argument(key)) {
@@ -237,6 +238,7 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 		return -EIO;
 	}
 
+	/* This has no effect on newer (2012) SMCs */
 	if (send_byte(len, APPLESMC_DATA_PORT)) {
 		pr_warn("%.4s: read len fail\n", key);
 		return -EIO;
@@ -250,6 +252,17 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 		buffer[i] = inb(APPLESMC_DATA_PORT);
 	}
 
+	/* Read the data port until bit0 is cleared */
+	for (i = 0; i < 16; i++) {
+		udelay(APPLESMC_MIN_WAIT);
+		status = inb(APPLESMC_CMD_PORT);
+		if (!(status & 0x01))
+			break;
+		data = inb(APPLESMC_DATA_PORT);
+	}
+	if (i)
+		pr_warn("flushed %d bytes, last value is: %d\n", i, data);
+
 	return 0;
 }
 
-- 
1.8.3.4

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