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Date:   Wed, 29 Mar 2023 10:24:58 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     gregory.greenman@intel.com, kvalo@kernel.org,
        ndesaulniers@google.com, trix@redhat.com, avraham.stern@intel.com,
        krishnanand.prabhu@intel.com, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, patches@lists.linux.dev,
        Arnd Bergmann <arnd@arndb.de>,
        "kernelci.org bot" <bot@kernelci.org>
Subject: Re: [PATCH wireless-next] wifi: iwlwifi: mvm: Avoid 64-bit division
 in iwl_mvm_get_crosstimestamp_fw()
Message-ID: <20230329172458.GA209886@dev-arch.thelio-3990X>
References: <20230329-iwlwifi-ptp-avoid-64-bit-div-v1-1-ad8db8d66bc2@kernel.org>
 <9058a032c177e9b04adbf944ad34c5ed8090d9d6.camel@sipsolutions.net>
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On Wed, Mar 29, 2023 at 07:20:43PM +0200, Johannes Berg wrote:
> 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.

Okay, that is good information, thanks for providing it!

> 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.

Right, I sent this version of the fix to spur discussion around whether
or not this was an acceptable approach, rather than having the question
sit unanswered in our issue tracker :) I have no problems sending a v2
to use div_u64() and be done with it, which I will do shortly.

Thanks for the quick input, cheers!
Nathan