netdev - Re: [PATCH net-next 1/2] net: dsa: mv88e6xxx: Improve performance of busy bit polling

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Date:   Thu, 27 Jan 2022 14:06:01 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Tobias Waldekranz <tobias@...dekranz.com>
Cc:     davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] net: dsa: mv88e6xxx: Improve performance of
 busy bit polling

On Thu, Jan 27, 2022 at 01:58:12PM +0100, Tobias Waldekranz wrote:
> On Thu, Jan 27, 2022 at 00:45, Andrew Lunn <andrew@...n.ch> wrote:
> > There are a few bit-banging systems out there. For those, i wonder if
> > 50ms is too short? With the old code, they had 16 chances, no matter
> > how slow they were. With the new code, if they take 50ms for one
> > transaction, they don't get a second chance.
> >
> > But if they have taken 50ms, around 37ms has been spent with the
> > preamble, start, op, phy address, and register address. I assume at
> > that point the switch actually looks at the register, and given your
> > timings, it really should be ready, so a second loop is probably not
> > required?
> >
> > O.K, so this seems safe.
> 
> I think you raise a good point though. Say that you then have this
> series of events:
> 
> 1. Bang out ST
> 2. Bang out OP
> 3. Bang out PHYADR
> 4. Bang out REGADR
> 5. Clock out TA
> 6. schedule()
> 7. A SCHED_FIFO/P99 task runs
> 8. Clock in DATA
> 
> - Steps 1 through 5 could plausibly be completed before the bit clears
>   if you are running over some memory mapped GPIO lines
> - Step 7 could execute for more than 50ms
> - After step 8, you would see the busy bit set, but your time is up

So this is the opposite case i was thinking about. A very fast bit
banger. Yes, in theory this could happen.

> All of this is of course _very_ unlikely, but not impossible. Should we
> ensure that you always get at least two bites at the apple?

This is why i always point people at include/linux/iopoll.h. It
handles conditions like this by doing one more poll after the timeout
just to be sure the scheduler has not interfered. So a minimum of 2
would be good.

      Andrew