get_rtc_noop() is used when an RTC is unavailable, but leaves its output
argument untouched. When CONFIG_INIT_STACK_ALL_ZERO is not enabled,
timekeeping_init() may therefore receive a nonzero invalid persistent
clock value and warn:
"Persistent clock returned invalid value"

Initialize the output to zero, the documented representation of an
unsupported persistent clock.

Fixes: c311ed6183f4 ("x86/init: Allow DT configured systems to disable RTC at boot time")
Cc: stable@vger.kernel.org
Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
---
 arch/x86/kernel/x86_init.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
index 252c5827d0634..3b9b41820c84c 100644
--- a/arch/x86/kernel/x86_init.c
+++ b/arch/x86/kernel/x86_init.c
@@ -37,7 +37,7 @@ static void iommu_shutdown_noop(void) { }
 bool __init bool_x86_init_noop(void) { return false; }
 void x86_op_int_noop(int cpu) { }
 int set_rtc_noop(const struct timespec64 *now) { return -EINVAL; }
-void get_rtc_noop(struct timespec64 *now) { }
+void get_rtc_noop(struct timespec64 *now) { *now = (struct timespec64){0}; }
 
 static __initconst const struct of_device_id of_cmos_match[] = {
 	{ .compatible = "motorola,mc146818" },
-- 
2.43.0

On Mon, Aug 31, 2026 at 08:25:28AM +0000, Naman Jain wrote:
> get_rtc_noop() is used when an RTC is unavailable, but leaves its output
> argument untouched. When CONFIG_INIT_STACK_ALL_ZERO is not enabled,
> timekeeping_init() may therefore receive a nonzero invalid persistent
> clock value and warn:
> "Persistent clock returned invalid value"
> 
> Initialize the output to zero, the documented representation of an
> unsupported persistent clock.
> 
> Fixes: c311ed6183f4 ("x86/init: Allow DT configured systems to disable RTC at boot time")
> Cc: stable@vger.kernel.org
> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
> ---
>  arch/x86/kernel/x86_init.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
> index 252c5827d0634..3b9b41820c84c 100644
> --- a/arch/x86/kernel/x86_init.c
> +++ b/arch/x86/kernel/x86_init.c
> @@ -37,7 +37,7 @@ static void iommu_shutdown_noop(void) { }
>  bool __init bool_x86_init_noop(void) { return false; }
>  void x86_op_int_noop(int cpu) { }
>  int set_rtc_noop(const struct timespec64 *now) { return -EINVAL; }
> -void get_rtc_noop(struct timespec64 *now) { }
> +void get_rtc_noop(struct timespec64 *now) { *now = (struct timespec64){0}; }

Hi Naman,

Could be

*now = (struct timespec64){};

to save a byte :)

LGTM.
Reviewed-by: Roman Kisel <vdso@mailbox.org>

>  
>  static __initconst const struct of_device_id of_cmos_match[] = {
>  	{ .compatible = "motorola,mc146818" },
> -- 
> 2.43.0
>

On 9/12/2026 9:05 AM, Roman Kisel wrote:
> On Mon, Aug 31, 2026 at 08:25:28AM +0000, Naman Jain wrote:
>> get_rtc_noop() is used when an RTC is unavailable, but leaves its output
>> argument untouched. When CONFIG_INIT_STACK_ALL_ZERO is not enabled,
>> timekeeping_init() may therefore receive a nonzero invalid persistent
>> clock value and warn:
>> "Persistent clock returned invalid value"
>>
>> Initialize the output to zero, the documented representation of an
>> unsupported persistent clock.
>>
>> Fixes: c311ed6183f4 ("x86/init: Allow DT configured systems to disable RTC at boot time")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
>> ---
>>   arch/x86/kernel/x86_init.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
>> index 252c5827d0634..3b9b41820c84c 100644
>> --- a/arch/x86/kernel/x86_init.c
>> +++ b/arch/x86/kernel/x86_init.c
>> @@ -37,7 +37,7 @@ static void iommu_shutdown_noop(void) { }
>>   bool __init bool_x86_init_noop(void) { return false; }
>>   void x86_op_int_noop(int cpu) { }
>>   int set_rtc_noop(const struct timespec64 *now) { return -EINVAL; }
>> -void get_rtc_noop(struct timespec64 *now) { }
>> +void get_rtc_noop(struct timespec64 *now) { *now = (struct timespec64){0}; }
> 
> Hi Naman,
> 
> Could be
> 
> *now = (struct timespec64){};
> 
> to save a byte :)

hi Roman,
Thanks for reviewing.

Tianyu pointed out offline that this is handled in yet another way in 
jailhouse code:

arch/x86/kernel/jailhouse.c
static void jailhouse_get_wallclock(struct timespec64 *now)
{
       memset(now, 0, sizeof(now));
}

Regards,
Naman

> 
> LGTM.
> Reviewed-by: Roman Kisel <vdso@mailbox.org>
> 
>>   
>>   static __initconst const struct of_device_id of_cmos_match[] = {
>>   	{ .compatible = "motorola,mc146818" },
>> -- 
>> 2.43.0
>>

> On 09/14/2026 4:31 AM PDT Naman Jain <namjain@linux.microsoft.com> wrote:
> 
>  
> On 9/12/2026 9:05 AM, Roman Kisel wrote:
> > On Mon, Aug 31, 2026 at 08:25:28AM +0000, Naman Jain wrote:

[...]

> >> +void get_rtc_noop(struct timespec64 *now) { *now = (struct timespec64){0}; }
> > 
> > Hi Naman,
> > 
> > Could be
> > 
> > *now = (struct timespec64){};
> > 
> > to save a byte :)
> 
> hi Roman,
> Thanks for reviewing.
> 
> Tianyu pointed out offline that this is handled in yet another way in 
> jailhouse code:
> 
> arch/x86/kernel/jailhouse.c
> static void jailhouse_get_wallclock(struct timespec64 *now)
> {
>        memset(now, 0, sizeof(now));
> }

Appreciate sharing! Makes sense to me, looks way more explicit. Most
importantly imo, the memset isn't actually producing a call here so
the clock no-op stays cheap when choosing `memset` over ` = {}`.

One can see that the kernel build compiles ` = {0}`, ` = {}` into

pxor ...
movups ...

on x64 in the above. I made sure the memset "call" is relaxed/optimized
away into the same sequence `pxor ...; movups ...`.

> 
> Regards,
> Naman
> 
> > 
> > LGTM.
> > Reviewed-by: Roman Kisel <vdso@mailbox.org>
> > 
> >>   
> >>   static __initconst const struct of_device_id of_cmos_match[] = {
> >>   	{ .compatible = "motorola,mc146818" },
> >> -- 
> >> 2.43.0
> >>

Reviewed-by: Hardik Garg <hargar@linux.microsoft.com>