From: Johannes Berg <johannes@sipsolutions.net>
To: Nathan Chancellor <nathan@kernel.org>,
gregory.greenman@intel.com, kvalo@kernel.org
Cc: 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()
Date: Wed, 29 Mar 2023 19:20:43 +0200 [thread overview]
Message-ID: <9058a032c177e9b04adbf944ad34c5ed8090d9d6.camel@sipsolutions.net> (raw)
In-Reply-To: <20230329-iwlwifi-ptp-avoid-64-bit-div-v1-1-ad8db8d66bc2@kernel.org>
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
next prev parent reply other threads:[~2023-03-29 17:20 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-29 17:05 Nathan Chancellor
2023-03-29 17:20 ` Johannes Berg [this message]
2023-03-29 17:24 ` Nathan Chancellor
2023-03-29 17:30 ` Nick Desaulniers
2023-03-29 19:00 ` Johannes Berg
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