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From:   Petr Machata <petrm@mellanox.com>
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Randy Dunlap <rdunlap@infradead.org>,
        Jiri Pirko <jiri@mellanox.com>,
        Ido Schimmel <idosch@mellanox.com>,
        "David S. Miller" <davem@davemloft.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mlxsw: spectrum_qdisc: Fix 64-bit division error in mlxsw_sp_qdisc_tbf_rate_kbps
In-reply-to: <20200131014816.GA54472@ubuntu-x2-xlarge-x86>
Date:   Mon, 03 Feb 2020 11:03:42 +0100
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Nathan Chancellor <natechancellor@gmail.com> writes:

> 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@gmail.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.

Yes, that's the correct choice. Divide first, that way we can't
overflow.

Thanks for taking care of this.