linux-kernel - Re: [PATCH] ACPI: CPPC: Fix bit_offset shift in MASK

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Date: Wed, 17 Apr 2024 10:36:59 -0700
From: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
To: Jarred White <jarredwhite@...ux.microsoft.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>, 
	Easwar Hariharan <eahariha@...ux.microsoft.com>, "open list:ACPI" <linux-acpi@...r.kernel.org>, 
	open list <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] ACPI: CPPC: Fix bit_offset shift in MASK_VAL macro

Looks good to me.

Reviewed-by: Vanshidhar Konda <vanshikonda@...amperecomputing.com>

On Mon, Apr 08, 2024 at 10:23:09PM -0700, Jarred White wrote:
>Commit 2f4a4d63a193 ("ACPI: CPPC: Use access_width over bit_width for
>system memory accesses") neglected to properly wrap the bit_offset shift
>when it comes to applying the mask. This may cause incorrect values to be
>read and may cause the cpufreq module not be loaded.
>
>[   11.059751] cpu_capacity: CPU0 missing/invalid highest performance.
>[   11.066005] cpu_capacity: partial information: fallback to 1024 for all CPUs
>
>Also, corrected the bitmask generation in GENMASK (extra bit being added).
>
>Fixes: 2f4a4d63a193 ("ACPI: CPPC: Use access_width over bit_width for system memory accesses")
>Signed-off-by: Jarred White <jarredwhite@...ux.microsoft.com>
>CC: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
>CC: stable@...r.kernel.org #5.15+
>---
> drivers/acpi/cppc_acpi.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>index 4bfbe55553f4..00a30ca35e78 100644
>--- a/drivers/acpi/cppc_acpi.c
>+++ b/drivers/acpi/cppc_acpi.c
>@@ -170,8 +170,8 @@ show_cppc_data(cppc_get_perf_ctrs, cppc_perf_fb_ctrs, wraparound_time);
> #define GET_BIT_WIDTH(reg) ((reg)->access_width ? (8 << ((reg)->access_width - 1)) : (reg)->bit_width)
>
> /* Shift and apply the mask for CPC reads/writes */
>-#define MASK_VAL(reg, val) ((val) >> ((reg)->bit_offset & 			\
>-					GENMASK(((reg)->bit_width), 0)))
>+#define MASK_VAL(reg, val) (((val) >> (reg)->bit_offset) & 			\
>+					GENMASK(((reg)->bit_width) - 1, 0))
>
> static ssize_t show_feedback_ctrs(struct kobject *kobj,
> 		struct kobj_attribute *attr, char *buf)
>-- 
>2.33.8
>