netdev - Re: [PATCH wireless-next] wifi: iwlwifi: mvm: Avoid 64-bit division in iwl_mvm_get_crosstimestamp

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Date:   Wed, 29 Mar 2023 19:20:43 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Nathan Chancellor <nathan@...nel.org>, gregory.greenman@...el.com,
        kvalo@...nel.org
Cc:     ndesaulniers@...gle.com, trix@...hat.com, avraham.stern@...el.com,
        krishnanand.prabhu@...el.com, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, patches@...ts.linux.dev,
        Arnd Bergmann <arnd@...db.de>,
        "kernelci.org bot" <bot@...nelci.org>
Subject: Re: [PATCH wireless-next] wifi: iwlwifi: mvm: Avoid 64-bit division
 in iwl_mvm_get_crosstimestamp_fw()

On Wed, 2023-03-29 at 10:05 -0700, Nathan Chancellor wrote:
> 
> GCC has optimizations for division by a constant that clang does not
> implement, so this issue is not visible when building with GCC.

Huh yeah, we did 32-bit builds with gcc ...

> Using div_u64() would resolve this issue, but Arnd points out that this
> can be quite expensive and the timestamp is being read at nanosecond
> granularity. 

Doesn't matter though, all the calculations are based on just the
command response from the firmware, which (tries to) take it in a
synchronised fashion.

So taking more time here would be fine, as far as I can tell.

> Nick pointed out that the result of this division is being
> stored to a 32-bit type anyways, so truncate gp2_10ns first then do the
> division, which elides the need for libcalls.

That loses ~7 top bits though, no? I'd be more worried about that, than
the time div_u64() takes.

johannes