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Subject: [PATCH] applesmc: Re-work SMC comms v2
From:   Brad Campbell <brad@fnarfbargle.com>
To:     Guenter Roeck <linux@roeck-us.net>,
        Andreas Kemnade <andreas@kemnade.info>,
        Jean Delvare <jdelvare@suse.com>
Cc:     Arnd Bergmann <arnd@arndb.de>, rydberg@bitmath.org,
        linux-hwmon@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        hns@goldelico.com
References: <20200930105442.3f642f6c@aktux>
 <20200930164446.GB219887@roeck-us.net>
 <CAK8P3a2CbhJT+B-F+cnX+uiJep9oiLM28n045-ATaVaU41u2hw@mail.gmail.com>
 <20201002002251.28462e64@aktux>
 <7543ef85-727d-96c3-947e-5b18e9e6c44d@roeck-us.net>
 <20201006090226.4275c824@kemnade.info>
 <db042e9b-be41-11b1-7059-3881b1da5c8b@fnarfbargle.com>
 <68467f1b-cea1-47ea-a4d4-8319214b072a@fnarfbargle.com>
 <20201104142057.62493c12@aktux>
 <2436afef-99c6-c352-936d-567bf553388c@fnarfbargle.com>
 <7a085650-2399-08c0-3c4d-6cd1fa28a365@roeck-us.net>
 <fc36d066-c432-e7d2-312f-a0a592446fe2@fnarfbargle.com>
 <10027199-5d31-93e7-9bd8-7baaebff8b71@roeck-us.net>
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Message-ID: <3c72ccc3-4de1-b5d0-423d-7b8c80991254@fnarfbargle.com>
Date:   Thu, 5 Nov 2020 18:26:24 +1100
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Commit fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()") introduced
an issue whereby communication with the SMC became unreliable with write
errors like :

[  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.

v2 : Address logic and coding style

Reported-by: Andreas Kemnade <andreas@kemnade.info>
Fixes: fff2d0f701e6 ("hwmon: (applesmc) avoid overlong udelay()")
Signed-off-by: Brad Campbell <brad@fnarfbargle.com>

---
diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index a18887990f4a..de890f3ec12f 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -42,6 +42,11 @@
 
 #define APPLESMC_MAX_DATA_LENGTH 32
 
+/* Apple SMC status bits */
+#define SMC_STATUS_AWAITING_DATA  BIT(0) /* SMC has data waiting */
+#define SMC_STATUS_IB_CLOSED      BIT(1) /* Will ignore any input */
+#define SMC_STATUS_BUSY           BIT(2) /* Command in progress */
+
 /* wait up to 128 ms for a status change. */
 #define APPLESMC_MIN_WAIT	0x0010
 #define APPLESMC_RETRY_WAIT	0x0100
@@ -151,65 +156,69 @@ 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);
+		}
 	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;
+	int ret;
 
+	ret = wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY | SMC_STATUS_IB_CLOSED);
+	if (ret)
+		return ret;
 	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);
-	}
+	return wait_status(skip ? 0 : SMC_STATUS_BUSY, SMC_STATUS_BUSY);
+}
 
-	pr_warn("send_byte(0x%02x, 0x%04x) fail: 0x%02x\n", cmd, port, status);
-	return -EIO;
+static int send_byte(u8 cmd, u16 port)
+{
+	return send_byte_data(cmd, port, false);
 }
 
+/*
+ * 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;
+	int ret;
+
+	ret = wait_status(0, SMC_STATUS_IB_CLOSED);
+	if (ret)
+		return ret;
+
+	status = inb(APPLESMC_CMD_PORT);
+
+	outb(cmd, APPLESMC_CMD_PORT);
+	return wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY);
 }
 
 static int send_argument(const char *key)
@@ -239,7 +248,9 @@ static int read_smc(u8 cmd, const char *key, u8 *buffer, u8 len)
 	}
 
 	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 +261,7 @@ 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);
 	}
@@ -275,7 +286,7 @@ static int write_smc(u8 cmd, const char *key, const u8 *buffer, u8 len)
 	}
 
 	for (i = 0; i < len; i++) {
-		if (send_byte(buffer[i], APPLESMC_DATA_PORT)) {
+		if (send_byte_data(buffer[i], APPLESMC_DATA_PORT, i == len - 1)) {
 			pr_warn("%s: write data fail\n", key);
 			return -EIO;
 		}