linux-kernel - Re: [PATCH] mlxsw: spectrum_qdisc: Fix 64-bit division error in mlxsw_sp_qdisc_tbf_rate

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Date:   Thu, 30 Jan 2020 18:48:16 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Jiri Pirko <jiri@...lanox.com>, Ido Schimmel <idosch@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Petr Machata <petrm@...lanox.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mlxsw: spectrum_qdisc: Fix 64-bit division error in
 mlxsw_sp_qdisc_tbf_rate_kbps

On Thu, Jan 30, 2020 at 05:43:56PM -0800, Randy Dunlap wrote:
> On 1/30/20 3:26 PM, Nathan Chancellor wrote:
> > When building arm32 allmodconfig:
> > 
> > ERROR: "__aeabi_uldivmod"
> > [drivers/net/ethernet/mellanox/mlxsw/mlxsw_spectrum.ko] undefined!
> > 
> > rate_bytes_ps has type u64, we need to use a 64-bit division helper to
> > avoid a build error.
> > 
> > Fixes: a44f58c41bfb ("mlxsw: spectrum_qdisc: Support offloading of TBF Qdisc")
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c b/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
> > index 79a2801d59f6..65e681ef01e8 100644
> > --- a/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
> > +++ b/drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c
> > @@ -614,7 +614,7 @@ mlxsw_sp_qdisc_tbf_rate_kbps(struct tc_tbf_qopt_offload_replace_params *p)
> >  	/* TBF interface is in bytes/s, whereas Spectrum ASIC is configured in
> >  	 * Kbits/s.
> >  	 */
> > -	return p->rate.rate_bytes_ps / 1000 * 8;
> > +	return div_u64(p->rate.rate_bytes_ps, 1000 * 8);
> 
> not quite right AFAICT.
> 
> try either
> 	return div_u64(p->rate.rate_bytes_ps * 8, 1000);
> or
> 	return div_u64(p->rate.rate_bytes_ps, 1000) * 8;
> 

Gah, I swear I can math... Thank you for catching this, v2 incoming with
the later because I think it looks better.

Cheers,
Nathan

> >  }
> >  
> >  static int
> > 
> 
> 
> -- 
> ~Randy
>