applesmc: Re-work SMC comms v1

Commit fff2d0f ("hwmon: (applesmc) avoid overlong udelay()") introduced
an issue whereby communication with the SMC became unreliable with write
errors :

[  120.378614] applesmc: send_byte(0x00, 0x0300) fail: 0x40
[  120.378621] applesmc: LKSB: write data fail
[  120.512782] applesmc: send_byte(0x00, 0x0300) fail: 0x40
[  120.512787] applesmc: LKSB: write data fail

The original code appeared to be timing sensitive and was not reliable with
the timing changes in the aforementioned commit.

This patch re-factors the SMC communication to remove the timing
dependencies and restore function with the changes previously committed.

Reported-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Brad Campbell <brad@fnarfbargle.com>

drivers/hwmon/applesmc.c

@@ -42,6 +42,11 @@
 
 #define APPLESMC_MAX_DATA_LENGTH 32
 
+/* Apple SMC status bits from VirtualSMC */
+#define SMC_STATUS_AWAITING_DATA  0x01  ///< Data waiting to be read
+#define SMC_STATUS_IB_CLOSED      0x02  /// A write is pending / will ignore input
+#define SMC_STATUS_BUSY           0x04  ///< Busy in the middle of a command.
+
 /* wait up to 128 ms for a status change. */
 #define APPLESMC_MIN_WAIT	0x0010
 #define APPLESMC_RETRY_WAIT	0x0100
@@ -151,73 +156,86 @@ static unsigned int key_at_index;
 static struct workqueue_struct *applesmc_led_wq;
 
 /*
- * wait_read - Wait for a byte to appear on SMC port. Callers must
- * hold applesmc_lock.
+ * Wait for specific status bits with a mask on the SMC
+ * Used before and after writes, and before reads
  */
-static int wait_read(void)
+
+static int wait_status(u8 val, u8 mask)
 {
 	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
 	u8 status;
 	int us;
 
 	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
-		usleep_range(us, us * 16);
 		status = inb(APPLESMC_CMD_PORT);
-		/* read: wait for smc to settle */
-		if (status & 0x01)
+		if ((status & mask) == val)
 			return 0;
 		/* timeout: give up */
 		if (time_after(jiffies, end))
 			break;
-	}
-
-	pr_warn("wait_read() fail: 0x%02x\n", status);
+		usleep_range(us, us * 16);
+		}
+	pr_warn("wait_status timeout: 0x%02x, 0x%02x, 0x%02x\n", status, val, mask);
 	return -EIO;
 }
 
 /*
- * send_byte - Write to SMC port, retrying when necessary. Callers
+ * send_byte_data - Write to SMC data port. Callers
  * must hold applesmc_lock.
+ * Parameter skip must be true on the last write of any
+ * command or it'll time out.
  */
-static int send_byte(u8 cmd, u16 port)
+
+static int send_byte_data(u8 cmd, u16 port, bool skip)
 {
-	u8 status;
-	int us;
-	unsigned long end = jiffies + (APPLESMC_MAX_WAIT * HZ) / USEC_PER_SEC;
+	u8 wstat = SMC_STATUS_BUSY;
 
+	if (skip)
+		wstat = 0;
+	if (wait_status(SMC_STATUS_BUSY,
+	SMC_STATUS_BUSY | SMC_STATUS_IB_CLOSED))
+		goto fail;
 	outb(cmd, port);
-	for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
-		usleep_range(us, us * 16);
-		status = inb(APPLESMC_CMD_PORT);
-		/* write: wait for smc to settle */
-		if (status & 0x02)
-			continue;
-		/* ready: cmd accepted, return */
-		if (status & 0x04)
-			return 0;
-		/* timeout: give up */
-		if (time_after(jiffies, end))
-			break;
-		/* busy: long wait and resend */
-		udelay(APPLESMC_RETRY_WAIT);
-		outb(cmd, port);
-	}
-
-	pr_warn("send_byte(0x%02x, 0x%04x) fail: 0x%02x\n", cmd, port, status);
+	if (!wait_status(wstat,
+	SMC_STATUS_BUSY))
+		return 0;
+fail:
+	pr_warn("send_byte_data(0x%02x, 0x%04x) fail\n", cmd, APPLESMC_CMD_PORT);
 	return -EIO;
 }
 
+/*
+ * send_command - Write a command to the SMC. Callers must hold applesmc_lock.
+ * If SMC is in undefined state, any new command write resets the state machine.
+ */
+
 static int send_command(u8 cmd)
 {
-	return send_byte(cmd, APPLESMC_CMD_PORT);
+	u8 status;
+
+	if (wait_status(0,
+	SMC_STATUS_IB_CLOSED)) {
+		pr_warn("send_command SMC was busy\n");
+		goto fail; }
+
+	status = inb(APPLESMC_CMD_PORT);
+
+	outb(cmd, APPLESMC_CMD_PORT);
+	if (!wait_status(SMC_STATUS_BUSY,
+	SMC_STATUS_BUSY))
+		return 0;
+fail:
+	pr_warn("send_cmd(0x%02x, 0x%04x) fail\n", cmd, APPLESMC_CMD_PORT);
+	return -EIO;
 }
 
 static int send_argument(const char *key)
 {
 	int i;
 
 	for (i = 0; i < 4; i++)
-		if (send_byte(key[i], APPLESMC_DATA_PORT))
+	/* Parameter skip is false as we always send data after an argument */
+		if (send_byte_data(key[i], APPLESMC_DATA_PORT, false))
 			return -EIO;
 	return 0;
 }
@@ -233,13 +251,15 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 	}
 
 	/* This has no effect on newer (2012) SMCs */
-	if (send_byte(len, APPLESMC_DATA_PORT)) {
+	if (send_byte_data(len, APPLESMC_DATA_PORT, false)) {
 		pr_warn("%.4s: read len fail\n", key);
 		return -EIO;
 	}
 
 	for (i = 0; i < len; i++) {
-		if (wait_read()) {
+		if (wait_status(SMC_STATUS_AWAITING_DATA | SMC_STATUS_BUSY,
+		SMC_STATUS_AWAITING_DATA | SMC_STATUS_BUSY |
+		SMC_STATUS_IB_CLOSED)) {
 			pr_warn("%.4s: read data[%d] fail\n", key, i);
 			return -EIO;
 		}
@@ -250,7 +270,7 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 	for (i = 0; i < 16; i++) {
 		udelay(APPLESMC_MIN_WAIT);
 		status = inb(APPLESMC_CMD_PORT);
-		if (!(status & 0x01))
+		if (!(status & SMC_STATUS_AWAITING_DATA))
 			break;
 		data = inb(APPLESMC_DATA_PORT);
 	}
@@ -263,20 +283,21 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 static int write_smc(u8 cmd, const char *key, const u8 *buffer, u8 len)
 {
 	int i;
+	u8 end = len-1;
 
 	if (send_command(cmd) || send_argument(key)) {
 		pr_warn("%s: write arg fail\n", key);
 		return -EIO;
 	}
 
-	if (send_byte(len, APPLESMC_DATA_PORT)) {
+	if (send_byte_data(len, APPLESMC_DATA_PORT, false)) {
 		pr_warn("%.4s: write len fail\n", key);
 		return -EIO;
 	}
 
 	for (i = 0; i < len; i++) {
-		if (send_byte(buffer[i], APPLESMC_DATA_PORT)) {
-			pr_warn("%s: write data fail\n", key);
+		if (send_byte_data(buffer[i], APPLESMC_DATA_PORT, (i == end))) {
+			pr_warn("%s: write data fail at %i\n", key, i);
 			return -EIO;
 		}
 	}