hwmon: applesmc: lighter wait mechanism, drastic improvement

The read fail ratio is sensitive to the delay between the first byte
written and the first byte read; apparently the sensors cannot be rushed.
Increasing the minimum wait time, without changing the total wait time,
improves the fail ratio from a 8% chance that any of the sensors fails in
one read, down to 0.4%, on a Macbook Air.  On a Macbook Pro 3,1, the
effect is even more apparent.  By reducing the number of status polls, the
ratio is further improved to below 0.1%.  Finally, increasing the total
wait time brings the fail ratio down to virtually zero.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Tested-by: Bob McElrath <bob@mcelrath.org>
Cc: Nicolas Boichat <nicolas@boichat.ch>
Cc: "Mark M. Hoffman" <mhoffman@lightlink.com>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 10 deletions

diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index 59266ce60cc2..bc011da79e14 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -49,6 +49,9 @@
 
 #define APPLESMC_MAX_DATA_LENGTH 32
 
+#define APPLESMC_MIN_WAIT       0x0040
+#define APPLESMC_MAX_WAIT       0x8000
+
 #define APPLESMC_STATUS_MASK    0x0f
 #define APPLESMC_READ_CMD       0x10
 #define APPLESMC_WRITE_CMD      0x11
@@ -172,25 +175,25 @@ static unsigned int key_at_index;
 static struct workqueue_struct *applesmc_led_wq;
 
 /*
- * __wait_status - Wait up to 10ms for the status port to get a certain value
+ * __wait_status - Wait up to 32ms for the status port to get a certain value
  * (masked with 0x0f), returning zero if the value is obtained.  Callers must
  * hold applesmc_lock.
  */
 static int __wait_status(u8 val)
 {
-        unsigned int i;
+        int us;
 
         val = val & APPLESMC_STATUS_MASK;
 
-        for (i = 0; i < 1000; i++) {
+        for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
+                udelay(us);
                 if ((inb(APPLESMC_CMD_PORT) & APPLESMC_STATUS_MASK) == val) {
                         if (debug)
                                 printk(KERN_DEBUG
-                                                "Waited %d us for status %x\n",
-                                                i*10, val);
+                                        "Waited %d us for status %x\n",
+                                        2 * us - APPLESMC_MIN_WAIT, val);
                         return 0;
                 }
-                udelay(10);
         }
 
         printk(KERN_WARNING "applesmc: wait status failed: %x != %x\n",
@@ -206,13 +209,12 @@ static int __wait_status(u8 val)
  */
 static int send_command(u8 cmd)
 {
-        int i;
-        for (i = 0; i < 1000; i++) {
+        int us;
+        for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
                 outb(cmd, APPLESMC_CMD_PORT);
-                udelay(5);
+                udelay(us);
                 if ((inb(APPLESMC_CMD_PORT) & APPLESMC_STATUS_MASK) == 0x0c)
                         return 0;
-                udelay(5);
         }
         printk(KERN_WARNING "applesmc: command failed: %x -> %x\n",
                 cmd, inb(APPLESMC_CMD_PORT));