linux-kernel - Re: [PATCH 1/1] char: hpet: Remove unused local variable 'm' in hpet

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Date:   Fri, 16 Apr 2021 10:08:25 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Clemens Ladisch <clemens@...isch.de>,
        Arnd Bergmann <arnd@...db.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Nils Carlson <nils.carlson@...csson.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/1] char: hpet: Remove unused local variable 'm' in
 hpet_interrupt()



On 2021/4/15 22:53, Greg Kroah-Hartman wrote:
> On Thu, Apr 15, 2021 at 10:24:04PM +0800, Zhen Lei wrote:
>> Commit 273ef9509b79 ("drivers/char/hpet.c: fix periodic-emulation for
>> delayed interrupt") removed the reference to local variable 'm', but
>> forgot to remove the definition and assignment of it. Due to
>> read_counter() indirectly calls "read barrier", the performance is
>> slightly degraded.
>>
>> Since the following comments give some description based on 'm', so move
>> the assignment of 'm' into it.
>>
>> Fixes: 273ef9509b79 ("drivers/char/hpet.c: fix periodic-emulation for delayed interrupt")
>> Reported-by: Hulk Robot <hulkci@...wei.com>
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> ---
>>  drivers/char/hpet.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
>> index ed3b7dab678dbd1..46950a0cda181a1 100644
>> --- a/drivers/char/hpet.c
>> +++ b/drivers/char/hpet.c
>> @@ -156,14 +156,16 @@ static irqreturn_t hpet_interrupt(int irq, void *data)
>>  	 * This has the effect of treating non-periodic like periodic.
>>  	 */
>>  	if ((devp->hd_flags & (HPET_IE | HPET_PERIODIC)) == HPET_IE) {
>> -		unsigned long m, t, mc, base, k;
>> +		unsigned long t, mc, base, k;
>>  		struct hpet __iomem *hpet = devp->hd_hpet;
>>  		struct hpets *hpetp = devp->hd_hpets;
>>  
>>  		t = devp->hd_ireqfreq;
>> -		m = read_counter(&devp->hd_timer->hpet_compare);
>>  		mc = read_counter(&hpet->hpet_mc);
>> -		/* The time for the next interrupt would logically be t + m,
>> +		/*
>> +		 * m = read_counter(&devp->hd_timer->hpet_compare);
> 
> Why did you comment this out?
> 
> And are you sure that yuou are not required to actually read that
> counter, even if you do not do anything with the value?  Lots of
> hardware works in odd ways...
> 
> Have you tested and verified that this still works properly?

Sorry, I didn't actually test it. I didn't see any dependency on
this read operation for other members' reads and writes. If this
read operation is potentially required, hopefully there is an
explanatory note next to it.

> 
> thanks,
> 
> greg k-h
> 
> .
>