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

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Message-ID: <465e2312486da2c68f26811884474d080e906d87.camel@sipsolutions.net>
Date:   Wed, 29 Mar 2023 21:00:47 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>, gregory.greenman@...el.com,
        kvalo@...nel.org, 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>,
        Craig Topper <craig.topper@...ive.com>
Subject: Re: [PATCH wireless-next] wifi: iwlwifi: mvm: Avoid 64-bit division
 in iwl_mvm_get_crosstimestamp_fw()

On Wed, 2023-03-29 at 10:30 -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