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Message-ID: <20230711101233.GM41919@unreal>
Date: Tue, 11 Jul 2023 13:12:33 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Florian Kauer <florian.kauer@...utronix.de>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>, davem@...emloft.net,
kuba@...nel.org, pabeni@...hat.com, edumazet@...gle.com,
netdev@...r.kernel.org, kurt@...utronix.de,
vinicius.gomes@...el.com, muhammad.husaini.zulkifli@...el.com,
tee.min.tan@...ux.intel.com, aravindhan.gunasekaran@...el.com,
sasha.neftin@...el.com, Naama Meir <naamax.meir@...ux.intel.com>
Subject: Re: [PATCH net 5/6] igc: Fix launchtime before start of cycle
On Tue, Jul 11, 2023 at 10:37:48AM +0200, Florian Kauer wrote:
> On 11.07.23 09:09, Leon Romanovsky wrote:
> > On Mon, Jul 10, 2023 at 09:35:02AM -0700, Tony Nguyen wrote:
> >> From: Florian Kauer <florian.kauer@...utronix.de>
> >>
> >> It is possible (verified on a running system) that frames are processed
> >> by igc_tx_launchtime with a txtime before the start of the cycle
> >> (baset_est).
> >>
> >> However, the result of txtime - baset_est is written into a u32,
> >> leading to a wrap around to a positive number. The following
> >> launchtime > 0 check will only branch to executing launchtime = 0
> >> if launchtime is already 0.
> >>
> >> Fix it by using a s32 before checking launchtime > 0.
> >>
> >> Fixes: db0b124f02ba ("igc: Enhance Qbv scheduling by using first flag bit")
> >> Signed-off-by: Florian Kauer <florian.kauer@...utronix.de>
> >> Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de>
> >> Tested-by: Naama Meir <naamax.meir@...ux.intel.com>
> >> Signed-off-by: Tony Nguyen <anthony.l.nguyen@...el.com>
> >> ---
> >> drivers/net/ethernet/intel/igc/igc_main.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> >> index 5d24930fed8f..4855caa3bae4 100644
> >> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> >> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> >> @@ -1016,7 +1016,7 @@ static __le32 igc_tx_launchtime(struct igc_ring *ring, ktime_t txtime,
> >> ktime_t base_time = adapter->base_time;
> >> ktime_t now = ktime_get_clocktai();
> >> ktime_t baset_est, end_of_cycle;
> >> - u32 launchtime;
> >> + s32 launchtime;
> >
> > The rest of igc_tx_launchtime() function is very questionable,
> > as ktime_sub_ns() returns ktime_t which is s64.
> >
> > 1049 launchtime = ktime_sub_ns(txtime, baset_est);
> > 1050 if (launchtime > 0)
> > 1051 div_s64_rem(launchtime, cycle_time, &launchtime);
> > 1052 else
> > 1053 launchtime = 0;
> > 1054
> > 1055 return cpu_to_le32(launchtime);
> >
>
> If I understand correctly, ktime_sub_ns(txtime, baset_est) will only return
> something larger than s32 max if cycle_time is larger than s32 max and if that
> is the case everything will be broken anyway since the corresponding hardware
> register only holds 30 bits.
I suggest you to use proper variable types, what about the following
snippet?
ktime_t launchtime;
launchtime = ktime_sub_ns(txtime, baset_est);
WARN_ON(upper_32_bits(launchtime));
div_s64_rem(launchtime, cycle_time, &launchtime);
return cpu_to_le32(lower_32_bits(launchtime));
>
> However, I do not see on first inspection where that case is properly handled
> (probably just by rejecting the TAPRIO schedule).
>
> Can someone with more experience in that area please jump in?
>
> Thanks,
> Florian
>
> >
> >> s64 n;
> >>
> >> n = div64_s64(ktime_sub_ns(now, base_time), cycle_time);
> >> --
> >> 2.38.1
> >>
> >>
>
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