From: Johannes Berg <johannes@sipsolutions.net>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>,
gregory.greenman@intel.com, kvalo@kernel.org, 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>,
Craig Topper <craig.topper@sifive.com>
Subject: Re: [PATCH wireless-next] wifi: iwlwifi: mvm: Avoid 64-bit division in iwl_mvm_get_crosstimestamp_fw()
Date: Wed, 29 Mar 2023 21:00:47 +0200 [thread overview]
Message-ID: <465e2312486da2c68f26811884474d080e906d87.camel@sipsolutions.net> (raw)
In-Reply-To: <CAKwvOdnQ9feXGYV2CUyVwg-FNAOmb4HBmDxMg243v28DzSfLuA@mail.gmail.com>
On Wed, 2023-03-29 at 10:30 -0700, Nick Desaulniers wrote:
> >
> > > 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.
>
> The result is still stored in a u32; there is a loss of precision
> regardless of use of div_u64 or open coded binary operator /.
>
Right, obviously.
> So is
> the loss of precision before the division as tolerable as after the
> division?
For all I can tell this is meant to be 'gp2' with an additional lower
bits to reach a unit/granularity of 10ns, basically in FW something like
gp2_10ns = gp2 * 100 + subsampling_10ns_unit
(and gp2 in FW is a 32-bit value, so it rolls over eventually).
But I _think_ we want to make a proper division by 100 to obtain back
the original 'gp2' value here.
johannes
prev parent reply other threads:[~2023-03-29 19:00 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-29 17:05 [PATCH wireless-next] wifi: iwlwifi: mvm: Avoid 64-bit division in iwl_mvm_get_crosstimestamp_fw() Nathan Chancellor
2023-03-29 17:20 ` Johannes Berg
2023-03-29 17:24 ` Nathan Chancellor
2023-03-29 17:30 ` Nick Desaulniers
2023-03-29 19:00 ` Johannes Berg [this message]
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