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Subject: Re: [PATCH] applesmc: Re-work SMC comms v2
To:     Henrik Rydberg <rydberg@bitmath.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, linux-hwmon@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        hns@goldelico.com, Guenter Roeck <linux@roeck-us.net>,
        Andreas Kemnade <andreas@kemnade.info>,
        Jean Delvare <jdelvare@suse.com>
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From:   Brad Campbell <brad@fnarfbargle.com>
Message-ID: <26d3f4f5-2e9a-cd20-1531-74cf44ef738c@fnarfbargle.com>
Date:   Sun, 8 Nov 2020 11:09:31 +1100
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On 8/11/20 5:31 am, Henrik Rydberg wrote:
> On 2020-11-06 21:02, Henrik Rydberg wrote:
>>> So as it stands, it does not work at all. I will continue to check another machine, and see if I can get something working.
>>
>> On the MacBookAir3,1 the situation is somewhat better.
>>
>> The first three tree positions result in zero failures and 10 reads per second. The fourth yields zero failues and 11 reads per second, within the margin of similarity.
>>
>> So, the patch appears to have no apparent effect on the 3,1 series.
>>
>> Now onto fixing the 1,1 behavior.
> 
> Hi again,
> 
> This patch, v3, works for me, on both MBA1,1 and MBA3,1. Both machines yields 25 reads per second.
> 
> It turns out that the origin code has a case that was not carried over to the v2 patch; the command byte needs to be resent upon the wrong status code. I added that back. Also, there seems to be a basic response time that needs to be respected, so I added back a small fixed delay after each write operation. I also took the liberty to reduce the number of status reads, and clean up error handling. Checkpatch is happy with this version.
> 
> The code obviously needs to be retested on the other machines, but the logic still follows what you wrote, Brad, and I have also checked it against the VirtualSMC code. It appears to make sense, so hopefully there wont be additional issues.
> 
> Thanks,
> Henrik
> 

G'day Henrik,

Which kernel was this based on? It won't apply to my 5.9 tree.

I assume the sprinkling of udelay(APPLESMC_MIN_WAIT) means the SMC is
slow in getting its status register set up. Could we instead just put
a single one of those up-front in wait_status?

Any chance you could try this one? I've added a retry to send_command and 
added a single global APPLESMC_MIN_WAIT before each status read.

From looking at your modified send_command, it appears the trigger for a 
retry is sending a command and the SMC doing absolutely nothing. This
should do the same thing.

Interestingly enough, by adding the udelay to wait_status on my machine I've
gone from 24 reads/s to 50 reads/s.

I've left out the remainder of the cleanups. Once we get a minimally working
patch I was going to look at a few cleanups, and I have some patches pending
to allow writing to the SMC from userspace (for setting BCLM and BFCL mainly)


diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index a18887990f4a..2190de78b5f5 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -32,6 +32,7 @@
 #include <linux/hwmon.h>
 #include <linux/workqueue.h>
 #include <linux/err.h>
+#include <linux/bits.h>
 
 /* data port used by Apple SMC */
 #define APPLESMC_DATA_PORT	0x300
@@ -42,6 +43,11 @@
 
 #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 +157,73 @@ 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;
 
+	udelay(APPLESMC_MIN_WAIT);
 	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;
+		usleep_range(us, us * 16);
 	}
-
-	pr_warn("wait_read() fail: 0x%02x\n", status);
 	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);
+	int ret;
+	int i;
+		
+	for (i=0; i < 16; i++) {
+		ret = wait_status(0, SMC_STATUS_IB_CLOSED);
+		if (ret)
+			return ret;
+	
+		outb(cmd, APPLESMC_CMD_PORT);
+		ret = wait_status(SMC_STATUS_BUSY, SMC_STATUS_BUSY);
+		if (!ret)
+			return ret;
+	}
+	return -EIO;
 }
 
 static int send_argument(const char *key)
@@ -239,7 +253,9 @@ 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 +266,7 @@ 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 +291,7 @@ 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;
 		}