linux-kernel - Re: [PATCH 22/71] ncr5380: Eliminate selecting state

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Message-ID: <CAMuHMdXwckticSMQpK6Fmm-ACJtqebTZXT9YGvJ3v1RWDD0fdA@mail.gmail.com>
Date:	Sun, 29 Nov 2015 14:50:53 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Finn Thain <fthain@...egraphics.com.au>
Cc:	Ondrej Zary <linux@...nbow-software.org>,
	"James E.J. Bottomley" <JBottomley@...n.com>,
	Michael Schmitz <schmitzmic@...il.com>,
	"Linux/m68k" <linux-m68k@...r.kernel.org>,
	scsi <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 22/71] ncr5380: Eliminate selecting state

Hi Finn,

On Sun, Nov 29, 2015 at 11:25 AM, Finn Thain <fthain@...egraphics.com.au> wrote:
> On Sun, 29 Nov 2015, Geert Uytterhoeven wrote:
>> This still heavily depends on the processing time spent in
>> NCR5380_read(). You should never use a value derived from
>> loops_per_jiffy for a non-empty loop,
>
> Sure but the time-out condition isn't supposed to be precise.
> Plus/minus a jiffy is no problem. Plus/minus a second is no good.

If an ISA access takes 8 µs, while the CPU runs at 1 GHz, i.e. 500M loops/s,
the difference will be huge.

>> as it may take much longer. Always compare with an maximum end time
>> instead.
>
> That can't work with interrupts disabled, which was the problem I was
> trying to solve by use of loops_per_jiffy.

Then you indeed can't use jiffies.
Perhaps you can calibrate an NCR5380_read() loop at driver init time,
and use the calibration value later?

> NCR5380_poll_politely() in mainline doesn't work with interrupts disabled
> either, hence patch 21.
>
>> E.g.:
>>
>>         end = jiffies + 2;        /* 1 jiffie + 1 safeguard */
>>         do {
>>                  if ((NCR5380_read(reg1) & bit1) == val1)
>>                          return 0;
>>                  cpu_relax();
>>         } while (time_before(jiffies, end);
>>
>> And a similar loop for "Busy-wait for up to 20 ms".
>
> Interrupts may be disabled during that loop also. Please refer to (and/or
> respond to) patch 21, "ncr5380: Sleep when polling, if possible", in which
> these changes were made.

So the above loop may never terminate. Oops...

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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