2.6.33: Problem continues: WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80

2.6.33: Problem continues: WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80

[Justin Piszcz]

Hello,

Again, on the Intel DP55KG board:

# uname -a
Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux

[    1.237600] ------------[ cut here ]------------
[    1.237890] WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80()
[    1.238221] Hardware name:
[    1.238504] hpet: compare register read back failed.
[    1.238793] Modules linked in:
[    1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
[    1.239605] Call Trace:
[    1.239886]  <IRQ>  [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
[    1.240409]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
[    1.240699]  [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
[    1.240992]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
[    1.241281]  [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
[    1.241573]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
[    1.241859]  [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
[    1.246533]  [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
[    1.246826]  [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
[    1.247118]  [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
[    1.247407]  [<ffffffff81029f55>] ? handle_irq+0x15/0x20
[    1.247689]  [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
[    1.247976]  [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
[    1.248262]  <EOI>  [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
[    1.248796]  [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
[    1.249080] ---[ end trace db7f668fb6fef4e1 ]---

Is this something Intel has to fix or is it a bug in the kernel?

Justin.

Re: 2.6.33: Problem continues: WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80

[Thomas Backlund]

Justin Piszcz skrev 25.2.2010 01:37:
> Hello,
>
> Again, on the Intel DP55KG board:
>
> # uname -a
> Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux
>
> [ 1.237600] ------------[ cut here ]------------
> [ 1.237890] WARNING: at arch/x86/kernel/hpet.c:404
> hpet_next_event+0x70/0x80()
> [ 1.238221] Hardware name:
> [ 1.238504] hpet: compare register read back failed.
> [ 1.238793] Modules linked in:
> [ 1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
> [ 1.239605] Call Trace:
> [ 1.239886] <IRQ> [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
> [ 1.240409] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [ 1.240699] [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
> [ 1.240992] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [ 1.241281] [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
> [ 1.241573] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [ 1.241859] [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
> [ 1.246533] [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
> [ 1.246826] [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
> [ 1.247118] [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
> [ 1.247407] [<ffffffff81029f55>] ? handle_irq+0x15/0x20
> [ 1.247689] [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
> [ 1.247976] [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
> [ 1.248262] <EOI> [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
> [ 1.248796] [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
> [ 1.249080] ---[ end trace db7f668fb6fef4e1 ]---
>
> Is this something Intel has to fix or is it a bug in the kernel?
>

Here is a suggested fix:

http://marc.info/?l=linux-kernel&m=126531123524059&w=2

--
Thomas

Re: 2.6.33: Problem continues: WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80

[Justin Piszcz]

On Thu, 25 Feb 2010, Thomas Backlund wrote:

> Justin Piszcz skrev 25.2.2010 01:37:
>> Hello,
>> 
>> Again, on the Intel DP55KG board:
>> 
>> # uname -a
>> Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux
>> 
>> [ 1.237600] ------------[ cut here ]------------
>> [ 1.237890] WARNING: at arch/x86/kernel/hpet.c:404
>> hpet_next_event+0x70/0x80()
>> [ 1.238221] Hardware name:
>> [ 1.238504] hpet: compare register read back failed.
>> [ 1.238793] Modules linked in:
>> [ 1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
>> [ 1.239605] Call Trace:
>> [ 1.239886] <IRQ> [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
>> [ 1.240409] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
>> [ 1.240699] [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
>> [ 1.240992] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
>> [ 1.241281] [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
>> [ 1.241573] [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
>> [ 1.241859] [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
>> [ 1.246533] [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
>> [ 1.246826] [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
>> [ 1.247118] [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
>> [ 1.247407] [<ffffffff81029f55>] ? handle_irq+0x15/0x20
>> [ 1.247689] [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
>> [ 1.247976] [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
>> [ 1.248262] <EOI> [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
>> [ 1.248796] [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
>> [ 1.249080] ---[ end trace db7f668fb6fef4e1 ]---
>> 
>> Is this something Intel has to fix or is it a bug in the kernel?
>> 
>
> Here is a suggested fix:
>
> http://marc.info/?l=linux-kernel&m=126531123524059&w=2
>
> --
> Thomas
>

Hi,

This looks like what I want, will it be included in 2.6.33.1 or 2.6.34?

Justin.

Re: 2.6.33: Problem continues: WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80

[Pallipadi, Venkatesh]

On Wed, Feb 24, 2010 at 03:37:04PM -0800, Justin Piszcz wrote:
> Hello,
> 
> Again, on the Intel DP55KG board:
> 
> # uname -a
> Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux
> 
> [    1.237600] ------------[ cut here ]------------
> [    1.237890] WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80()
> [    1.238221] Hardware name:
> [    1.238504] hpet: compare register read back failed.
> [    1.238793] Modules linked in:
> [    1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
> [    1.239605] Call Trace:
> [    1.239886]  <IRQ>  [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
> [    1.240409]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [    1.240699]  [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
> [    1.240992]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [    1.241281]  [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
> [    1.241573]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [    1.241859]  [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
> [    1.246533]  [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
> [    1.246826]  [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
> [    1.247118]  [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
> [    1.247407]  [<ffffffff81029f55>] ? handle_irq+0x15/0x20
> [    1.247689]  [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
> [    1.247976]  [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
> [    1.248262]  <EOI>  [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
> [    1.248796]  [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
> [    1.249080] ---[ end trace db7f668fb6fef4e1 ]---
> 
> Is this something Intel has to fix or is it a bug in the kernel?

This is a chipset erratum.

Thomas: You mentioned we can retain this check only for known-buggy and
hpet debug kind of options. But here is the simple workaround patch for
this particular erratum.

x86: Erratum workaround for read after write of HPET comparator

Some chipsets have a erratum due to which read immediately following a
write of HPET comparator returns old comparator value instead of most
recently written value.

Erratum 15 in
"Intel I/O Controller Hub 9 (ICH9) Family Specification Update"
(http://www.intel.com/assets/pdf/specupdate/316973.pdf)

Workaround for the errata is to read the comparator twice if the first
one fails.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
---
 arch/x86/kernel/hpet.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index ad80a1c..7b0dd63 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -399,9 +399,15 @@ static int hpet_next_event(unsigned long delta,
 	 * 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.
 	 */
-	WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
+	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");
+	}
 
 	return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
 }
-- 
1.6.0.6

Re: 2.6.33: Problem continues: WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80

[Thomas Gleixner]

On Thu, 25 Feb 2010, Pallipadi, Venkatesh wrote:

> On Wed, Feb 24, 2010 at 03:37:04PM -0800, Justin Piszcz wrote:
> > Hello,
> > 
> > Again, on the Intel DP55KG board:
> > 
> > # uname -a
> > Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux
> > 
> > [    1.237600] ------------[ cut here ]------------
> > [    1.237890] WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80()
> > [    1.238221] Hardware name:
> > [    1.238504] hpet: compare register read back failed.
> > [    1.238793] Modules linked in:
> > [    1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
> > [    1.239605] Call Trace:
> > [    1.239886]  <IRQ>  [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
> > [    1.240409]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> > [    1.240699]  [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
> > [    1.240992]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> > [    1.241281]  [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
> > [    1.241573]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> > [    1.241859]  [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
> > [    1.246533]  [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
> > [    1.246826]  [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
> > [    1.247118]  [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
> > [    1.247407]  [<ffffffff81029f55>] ? handle_irq+0x15/0x20
> > [    1.247689]  [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
> > [    1.247976]  [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
> > [    1.248262]  <EOI>  [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
> > [    1.248796]  [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
> > [    1.249080] ---[ end trace db7f668fb6fef4e1 ]---
> > 
> > Is this something Intel has to fix or is it a bug in the kernel?
> 
> This is a chipset erratum.
> 
> Thomas: You mentioned we can retain this check only for known-buggy and
> hpet debug kind of options. But here is the simple workaround patch for
> this particular erratum.

Yes, that'd be the preferred way. I need to find the box which had the
problem (ATI chipset) and force the readback for that one. That saves
all other machines the extra hardware access.

Thanks,

	tglx

[tip:x86/urgent] x86, hpet: Erratum workaround for read after write of HPET comparator

[tip-bot for Pallipadi, Venkatesh]

Commit-ID:  8da854cb02156c90028233ae1e85ce46a1d3f82c
Gitweb:     http://git.kernel.org/tip/8da854cb02156c90028233ae1e85ce46a1d3f82c
Author:     Pallipadi, Venkatesh <venkatesh.pallipadi@intel.com>
AuthorDate: Thu, 25 Feb 2010 10:53:48 -0800
Committer:  H. Peter Anvin <hpa@zytor.com>
CommitDate: Thu, 1 Apr 2010 15:21:47 -0700

x86, hpet: Erratum workaround for read after write of HPET comparator

On Wed, Feb 24, 2010 at 03:37:04PM -0800, Justin Piszcz wrote:
> Hello,
>
> Again, on the Intel DP55KG board:
>
> # uname -a
> Linux host 2.6.33 #1 SMP Wed Feb 24 18:31:00 EST 2010 x86_64 GNU/Linux
>
> [    1.237600] ------------[ cut here ]------------
> [    1.237890] WARNING: at arch/x86/kernel/hpet.c:404 hpet_next_event+0x70/0x80()
> [    1.238221] Hardware name:
> [    1.238504] hpet: compare register read back failed.
> [    1.238793] Modules linked in:
> [    1.239315] Pid: 0, comm: swapper Not tainted 2.6.33 #1
> [    1.239605] Call Trace:
> [    1.239886]  <IRQ>  [<ffffffff81056c13>] ? warn_slowpath_common+0x73/0xb0
> [    1.240409]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [    1.240699]  [<ffffffff81056cb0>] ? warn_slowpath_fmt+0x40/0x50
> [    1.240992]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [    1.241281]  [<ffffffff81041ad0>] ? hpet_next_event+0x70/0x80
> [    1.241573]  [<ffffffff81079608>] ? tick_dev_program_event+0x38/0xc0
> [    1.241859]  [<ffffffff81078e32>] ? tick_handle_oneshot_broadcast+0xe2/0x100
> [    1.246533]  [<ffffffff8102a67a>] ? timer_interrupt+0x1a/0x30
> [    1.246826]  [<ffffffff81085499>] ? handle_IRQ_event+0x39/0xd0
> [    1.247118]  [<ffffffff81087368>] ? handle_edge_irq+0xb8/0x160
> [    1.247407]  [<ffffffff81029f55>] ? handle_irq+0x15/0x20
> [    1.247689]  [<ffffffff810294a2>] ? do_IRQ+0x62/0xe0
> [    1.247976]  [<ffffffff8146be53>] ? ret_from_intr+0x0/0xa
> [    1.248262]  <EOI>  [<ffffffff8102f277>] ? mwait_idle+0x57/0x80
> [    1.248796]  [<ffffffff8102645c>] ? cpu_idle+0x5c/0xb0
> [    1.249080] ---[ end trace db7f668fb6fef4e1 ]---
>
> Is this something Intel has to fix or is it a bug in the kernel?

This is a chipset erratum.

Thomas: You mentioned we can retain this check only for known-buggy and
hpet debug kind of options. But here is the simple workaround patch for
this particular erratum.

Some chipsets have a erratum due to which read immediately following a
write of HPET comparator returns old comparator value instead of most
recently written value.

Erratum 15 in
"Intel I/O Controller Hub 9 (ICH9) Family Specification Update"
(http://www.intel.com/assets/pdf/specupdate/316973.pdf)

Workaround for the errata is to read the comparator twice if the first
one fails.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
LKML-Reference: <20100225185348.GA9674@linux-os.sc.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@gmail.com>
Cc: <stable@kernel.org>
---
 arch/x86/kernel/hpet.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index ee4fa1b..3d422da 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -399,9 +399,15 @@ static int hpet_next_event(unsigned long delta,
 	 * 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.
 	 */
-	WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt,
+	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");
+	}
 
 	return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0;
 }

[tip:x86/timers] x86, hpet: Restrict read back to affected ATI chipsets

[tip-bot for Thomas Gleixner]

Commit-ID:  30a564be9d9554c168a654eddc2165869cc0d7bf
Gitweb:     http://git.kernel.org/tip/30a564be9d9554c168a654eddc2165869cc0d7bf
Author:     Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 13 Apr 2010 15:31:36 +0200
Committer:  H. Peter Anvin <hpa@zytor.com>
CommitDate: Wed, 28 Apr 2010 18:14:29 -0700

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>
---
 arch/x86/include/asm/hpet.h |    1 +
 arch/x86/kernel/hpet.c      |   29 +++++++++++++++++------------
 arch/x86/kernel/quirks.c    |    5 +++++
 3 files changed, 23 insertions(+), 12 deletions(-)

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 unsigned long force_hpet_address;
 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 @@ 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;
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 @@ void force_hpet_resume(void)
 /*
  * 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,