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Subject: Re: [PATCH v2 07/10] clocksource/drivers/hyper-v: Handle vDSO
 differences inline
To:     Michael Kelley <mikelley@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>, "hpa@zytor.com" <hpa@zytor.com>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
References: <1614561332-2523-1-git-send-email-mikelley@microsoft.com>
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From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <f09da965-aeda-7edf-722c-dbc9d7daab38@linaro.org>
Date:   Tue, 2 Mar 2021 14:01:24 +0100
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On 02/03/2021 02:29, Michael Kelley wrote:
> From: Daniel Lezcano <daniel.lezcano@linaro.org> Sent: Monday, March 1, 2021 4:22 AM
>>
>> On 01/03/2021 02:15, Michael Kelley wrote:
>>> While the driver for the Hyper-V Reference TSC and STIMERs is architecture
>>> neutral, vDSO is implemented for x86/x64, but not for ARM64.  Current code
>>> calls into utility functions under arch/x86 (and coming, under arch/arm64)
>>> to handle the difference.
>>>
>>> Change this approach to handle the difference inline based on whether
>>> VDSO_CLOCK_MODE_HVCLOCK is present.  The new approach removes code under
>>> arch/* since the difference is tied more to the specifics of the Linux
>>> implementation than to the architecture.
>>>
>>> No functional change.
>>
>> A suggestion below
>>
>>
>>> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
>>> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
>>> ---
>>>  arch/x86/include/asm/mshyperv.h    |  4 ----
>>>  drivers/clocksource/hyperv_timer.c | 10 ++++++++--
>>>  2 files changed, 8 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
>>> index c73c127..5e5e08aa 100644
>>> --- a/drivers/clocksource/hyperv_timer.c
>>> +++ b/drivers/clocksource/hyperv_timer.c
>>> @@ -372,7 +372,9 @@ static void resume_hv_clock_tsc(struct clocksource *arg)
>>>
>>>  static int hv_cs_enable(struct clocksource *cs)
>>
>> static __maybe_unused int hv_cs_enable(struct clocksource *cs)
>>
>>>  {
>>> -	hv_enable_vdso_clocksource();
>>> +#ifdef VDSO_CLOCKMODE_HVCLOCK
>>> +	vclocks_set_used(VDSO_CLOCKMODE_HVCLOCK);
>>> +#endif
>>>  	return 0;
>>>  }
>>>
>>> @@ -385,6 +387,11 @@ static int hv_cs_enable(struct clocksource *cs)
>>>  	.suspend= suspend_hv_clock_tsc,
>>>  	.resume	= resume_hv_clock_tsc,
>>>  	.enable = hv_cs_enable,
>>> +#ifdef VDSO_CLOCKMODE_HVCLOCK
>>> +	.vdso_clock_mode = VDSO_CLOCKMODE_HVCLOCK,
>>> +#else
>>> +	.vdso_clock_mode = VDSO_CLOCKMODE_NONE,
>>> +#endif
>>
>> #ifdef VDSO_CLOCKMODE_HVCLOCK
>> 	.enable = hv_cs_enable,
>> 	.vdso_clock_mode = VDSO_CLOCKMODE_HVCLOCK,
>> #else
>> 	.vdso_clock_mode = VDSO_CLOCKMODE_NONE,
>> #endif
>>
> 
> Is there any particular benefit (that I might not be recognizing)
> to having the .enable function be NULL vs. a function that
> does nothing?  I can see the handful of places where the
> .enable function is invoked, and there doesn't seem to be
> much difference.
> 
> In any case, I have no problem with making the change in
> a v3 of the patch set.

It is just coding style, it allows to remove a #ifdef in the code.


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