netdev - Re: [PATCH net-next 1/2] uapi: ioam: Insertion frequency

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Message-ID: <873661285.9347975.1643538017855.JavaMail.zimbra@uliege.be>
Date:   Sun, 30 Jan 2022 11:20:17 +0100 (CET)
From:   Justin Iurman <justin.iurman@...ege.be>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org
Subject: Re: [PATCH net-next 1/2] uapi: ioam: Insertion frequency

>>> diff --git a/include/uapi/linux/ioam6_iptunnel.h
>>> b/include/uapi/linux/ioam6_iptunnel.h
>>> index 829ffdfcacca..462758cdba14 100644
>>> --- a/include/uapi/linux/ioam6_iptunnel.h
>>> +++ b/include/uapi/linux/ioam6_iptunnel.h
>>> @@ -30,6 +30,15 @@ enum {
>>>  enum {
>>>  	IOAM6_IPTUNNEL_UNSPEC,
>>>  
>>> +	/* Insertion frequency:
>>> +	 * "k over n" packets (0 < k <= n)
>>> +	 * [0.0001% ... 100%]
>>> +	 */
>>> +#define IOAM6_IPTUNNEL_FREQ_MIN 1
>>> +#define IOAM6_IPTUNNEL_FREQ_MAX 1000000
>> 
>> If min is 1 why not make the value unsigned?
> 
> The atomic_t type is just a wrapper for a signed int, so I didn't want
> to have to convert from signed to unsigned. I agree it'd sound better to
> have unsigned here, though.

Sorry, I figured out a cast is fine thanks to [1] which says:

"[...] the kernel uses -fno-strict-overflow
(which implies -fwrapv) and defines signed overflow to behave like
2s-complement.
Therefore, an explicitly unsigned variant of the atomic ops is strictly
unnecessary and we can simply cast, there is no UB. [...]"

  [1] https://www.kernel.org/doc/Documentation/atomic_t.txt