Re: [PATCH v2] net: ocelot: fix wrong time_after usage

[Clément Léger]

Le Sat, 21 May 2022 16:21:09 +0000,
Vladimir Oltean <vladimir.oltean@xxxxxxx> a écrit :

> On Sat, May 21, 2022 at 03:55:30PM +0200, Andrew Lunn wrote:
> > On Sat, May 21, 2022 at 12:31:15AM +0300, Pavel Skripkin wrote:  
> > > Accidentally noticed, that this driver is the only user of
> > > while (time_after(jiffies...)).
> > > 
> > > It looks like typo, because likely this while loop will finish after 1st
> > > iteration, because time_after() returns true when 1st argument _is after_
> > > 2nd one.
> > > 
> > > There is one possible problem with this poll loop: the scheduler could put
> > > the thread to sleep, and it does not get woken up for
> > > OCELOT_FDMA_CH_SAFE_TIMEOUT_US. During that time, the hardware has done
> > > its thing, but you exit the while loop and return -ETIMEDOUT.
> > > 
> > > Fix it by using sane poll API that avoids all problems described above
> > > 
> > > Fixes: 753a026cfec1 ("net: ocelot: add FDMA support")
> > > Suggested-by: Andrew Lunn <andrew@xxxxxxx>
> > > Signed-off-by: Pavel Skripkin <paskripkin@xxxxxxxxx>
> > > ---
> > > 
> > > I can't say if 0 is a good choise for 5th readx_poll_timeout() argument,
> > > so this patch is build-tested only.  
> >   
> > > Testing and suggestions are welcomed!  
> > 
> > If you had the hardware, i would suggest you profile how often it does
> > complete on the first iteration. And when it does not complete on the
> > first iteration, how many more iterations it needs.
> > 
> > Tobias made an interesting observation with the mv88e6xxx switch. He
> > found that two tight polls was enough 99% of the time. Putting a sleep
> > in there doubles the time it took to setup the switch. So he ended up
> > with a hybrid of open coded polling twice, followed by iopoll with a
> > timer value set.
> > 
> > That was with a heavily used poll function. How often is this function
> > used? No point in overly optimising this if it is not used much.  
> 
> If you're looking at me, I don't have the hardware to test, sorry.
> Frame DMA is one of the components NXP removed when building their DSA
> variants of these switches. But the function is called once or twice per
> NAPI poll cycle, so it's worth optimizing as much as possible.
> 
> Clement, could you please do some testing? The patch that Andrew is
> talking about is 35da1dfd9484 ("net: dsa: mv88e6xxx: Improve performance
> of busy bit polling").

So I actually tested and added logging to see if the CH_SAFE
register bits are set for the channel on the first iteration. From
what I could test (iperf3 with huge/non huge packets, TCP/UDP), it
always return true on the first try. So since I think Pavel solution
is ok to go with.

However, since ocelot_fdma_wait_chan_safe() is also called in the napi
poll function of this driver, I don't think sleeping is allowed (softirq
context) and thus I would suggest using the readx_poll_timeout_atomic()
function instead.

Regarding the delay to wait between each read, I don't have any
information about that possible value, the datasheet only says "wait
for the bit to be set" so I guess we'll have to live with an
approximate value.

Thanks,

-- 
Clément Léger,
Embedded Linux and Kernel engineer at Bootlin
https://bootlin.com