linux-kernel - Re: [OPW kernel] [PATCH v3] drivers: s390: net: ctcm: migrate variables to handle y2038 problem

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Date:	Mon, 03 Nov 2014 10:40:30 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	opw-kernel@...glegroups.com
Cc:	Aya Mahfouz <mahfouz.saif.elyazal@...il.com>,
	ursula.braun@...ibm.com, blaschka@...ux.vnet.ibm.com,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	linux390@...ibm.com, linux-s390@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [OPW kernel] [PATCH v3] drivers: s390: net: ctcm: migrate variables to handle y2038 problem

On Saturday 01 November 2014 02:16:14 Aya Mahfouz wrote:
> This patch is concerned with migrating the time variables for the s390
> network driver. The changes handle the y2038 problem where timespec will
> overflow in the year 2038. timespec was replaced by unsigned long and
> all time variables get their values from the jiffies global variable.
> This was done for the sake of speed and efficiency.
> 
> Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@...il.com>
> ---
> v1: Arnd has advised me to provide you with options for time
>     calculation. The first option: "accuracy" is used in this
>     patch. The second option: "speed" can be done through
>     jiffies.
> 
> v2: Moved on to the speed option. Let me know if I explicitly
>     need to include the jiffies header. The module compiles with
>     no problems on my side.
> 
> v3: Handled the error pointed out by Ursula. The current version
>     does not handle overflows. There are two solutions for this.
>     The first is to use jiffies_64 since s/390 is a 64-bit
>     architecture after all. The second is to use wrapper functions
>     like time_before and time_after. Two files were added too. 
>     They are: ctcm_main.c and netiucv.c. This patch could be sent
>     as a patchset in its final version for convenience. 

Hi Aya,

The patch looks correct to me now,

Reviewed-by: Arnd Bergmann <arnd@...db.de>

There is one small change that you could do for efficiency:

>  
> -	duration =
> -	    (done_stamp.tv_sec - ch->prof.send_stamp.tv_sec) * 1000000 +
> -	    (done_stamp.tv_nsec - ch->prof.send_stamp.tv_nsec) / 1000;
> +	duration =  jiffies_to_usecs(done_stamp - ch->prof.send_stamp);
>  	if (duration > ch->prof.tx_time)
>  		ch->prof.tx_time = duration;
>  

This means you are doing the jiffies_to_usecs conversion for every
packet. s390 has a fast multiplication instruction, so it won't matter
in practice, but if you do more conversions like this, a better approach
is to store the 'jiffies' value in ch->prof.tx_time and only convert
it to another unit when printing the number.

	Arnd
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