linux-kernel - [PATCH 1/1] Speedfreq-SMI call clobbers ECX

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Date:	Wed, 5 Mar 2008 15:59:09 +0100
From:	Stephan Diestelhorst <langer_mann@....de>
To:	davej@...emonkey.org.uk
Cc:	cpufreq@...ts.linux.org.uk, linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] Speedfreq-SMI call clobbers ECX

Dear Dave,
  I have found that using SMI to change the cpu's frequency on my DELL 
Latitude L400 clobbers the ECX register in speedstep_set_state, causing 
unneccessary retries because the "state" variable has changed silently (GCC 
assumes it is still present in ECX).

The patch is simple: Introduce temporary output "clobber_ecx" operand that 
consumes the clobbered value.

Cheers,
  Stephan

Signed-off: Stephan Diestelhorst <stephan.diestelhorst@...il.com>

--

--- linux-2.6.24.3/arch/x86/kernel/cpu/cpufreq/speedstep-smi.c.orig	2008-02-26 
01:20:20.000000000 +0100
+++ linux-2.6.24.3/arch/x86/kernel/cpu/cpufreq/speedstep-smi.c	2008-03-05 
15:56:42.000000000 +0100
@@ -160,7 +160,7 @@ static int speedstep_get_state (void)
  */
 static void speedstep_set_state (unsigned int state)
 {
-	unsigned int result = 0, command, new_state;
+	unsigned int result = 0, command, new_state, ecx_clobber;
 	unsigned long flags;
 	unsigned int function=SET_SPEEDSTEP_STATE;
 	unsigned int retry = 0;
@@ -184,7 +184,7 @@ static void speedstep_set_state (unsigne
 		__asm__ __volatile__(
 			"movl $0, %%edi\n"
 			"out %%al, (%%dx)\n"
-			: "=b" (new_state), "=D" (result)
+			: "=b" (new_state), "=D" (result), "=c" (ecx_clobber)
 			: "a" (command), "b" (function), "c" (state), "d" (smi_port), "S" (0)
 			);
 	} while ((new_state != state) && (retry <= SMI_TRIES));
@@ -195,7 +195,7 @@ static void speedstep_set_state (unsigne
 	if (new_state == state) {
 		dprintk("change to %u MHz succeeded after %u tries with result %u\n", 
(speedstep_freqs[new_state].frequency / 1000), retry, result);
 	} else {
-		printk(KERN_ERR "cpufreq: change failed with new_state %u and result %u\n", 
new_state, result);
+		printk(KERN_ERR "cpufreq: change to state %u failed with new_state %u and 
result %u\n", state, new_state, result);
 	}
 
 	return;
--
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