x86, hpet: Restrict read back to affected ATI chipsets

After programming the HPET, we do a readback as a workaround for
ATI/SBx00 chipsets as a synchronization.  Unfortunately this triggers
an erratum in newer ICH chipsets (ICH9+) where reading the comparator
immediately after the write returns the old value.  Furthermore, as
always, I/O reads are bad for performance.

Therefore, restrict the readback to the chipsets that need it, or, for
debugging purposes, when we are running with hpet=verbose.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Venkatesh Pallipadi <venki@google.com>
LKML-Reference: <20100225185348.GA9674@linux-os.sc.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 17 insertions, 12 deletions

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 23b4ecdffa9b..a198b7c87a12 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -36,6 +36,7 @@
 unsigned long				hpet_address;
 u8					hpet_blockid; /* OS timer block num */
 u8					hpet_msi_disable;
+u8					hpet_readback_cmp;
 
 #ifdef CONFIG_PCI_MSI
 static unsigned long			hpet_num_timers;
@@ -395,19 +396,23 @@ static int hpet_next_event(unsigned long delta,
 	 * at that point and we would wait for the next hpet interrupt
 	 * forever. We found out that reading the CMP register back
 	 * forces the transfer so we can rely on the comparison with
-	 * the counter register below. If the read back from the
-	 * compare register does not match the value we programmed
-	 * then we might have a real hardware problem. We can not do
-	 * much about it here, but at least alert the user/admin with
-	 * a prominent warning.
-	 * An erratum on some chipsets (ICH9,..), results in comparator read
-	 * immediately following a write returning old value. Workaround
-	 * for this is to read this value second time, when first
-	 * read returns old value.
+	 * the counter register below.
+	 *
+	 * That works fine on those ATI chipsets, but on newer Intel
+	 * chipsets (ICH9...) this triggers due to an erratum: Reading
+	 * the comparator immediately following a write is returning
+	 * the old value.
+	 *
+	 * We restrict the read back to the affected ATI chipsets (set
+	 * by quirks) and also run it with hpet=verbose for debugging
+	 * purposes.
 	 */
-	if (unlikely((u32)hpet_readl(HPET_Tn_CMP(timer)) != cnt)) {
-		WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
-		  KERN_WARNING "hpet: compare register read back failed.\n");
+	if (hpet_readback_cmp || hpet_verbose) {
+		u32 cmp = hpet_readl(HPET_Tn_CMP(timer));
+
+		if (cmp != cnt)
+			printk_once(KERN_WARNING
+			    "hpet: compare register read back failed.\n");
 	}
 
 	return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;