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>
diff --git a/arch/x86/include/asm/hpet.h b/arch/x86/include/asm/hpet.h
index 1d5c08a..004e6e2 100644
--- a/arch/x86/include/asm/hpet.h
+++ b/arch/x86/include/asm/hpet.h
@@ -68,6 +68,7 @@
extern u8 hpet_blockid;
extern int hpet_force_user;
extern u8 hpet_msi_disable;
+extern u8 hpet_readback_cmp;
extern int is_hpet_enabled(void);
extern int hpet_enable(void);
extern void hpet_disable(void);
diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 23b4ecd..a198b7c 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 @@
* 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;
diff --git a/arch/x86/kernel/quirks.c b/arch/x86/kernel/quirks.c
index 12e9fea..cd2c336 100644
--- a/arch/x86/kernel/quirks.c
+++ b/arch/x86/kernel/quirks.c
@@ -495,10 +495,15 @@
/*
* HPET MSI on some boards (ATI SB700/SB800) has side effect on
* floppy DMA. Disable HPET MSI on such platforms.
+ *
+ * Also force the read back of the CMP register in hpet_next_event()
+ * to work around the problem that the CMP register write seems to be
+ * delayed. See hpet_next_event() for details.
*/
static void force_disable_hpet_msi(struct pci_dev *unused)
{
hpet_msi_disable = 1;
+ hpet_readback_cmp = 1;
}
DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_ATI, PCI_DEVICE_ID_ATI_SBX00_SMBUS,