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 13:58:12 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Andrew Lunn <andrew@...n.ch>
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 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

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?