Date: Thu, 31 Aug 1995 05:17:15 +1000 From: Bruce Evans <bde@zeta.org.au> To: hackers@freebsd.org, jdl@chrome.onramp.net Subject: Re: More ATAPI -- possible insight into probe problems...? Message-ID: <199508301917.FAA18876@godzilla.zeta.org.au>
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>Oh my! Let's peek at my highly superior wd.c:
>...
> #ifdef WDDEBUG_PROBE
> printf("wd%d: checking RW register on port 0x%x + 0x%x\n",
> unit, du->dk_port, wd_cyl_lo);
> #endif
> /* check if we have registers that work */
> outb(du->dk_port + wd_cyl_lo, 0xa5); /* wd_cyl_lo is read/write */
> if (inb(du->dk_port + wd_cyl_lo) == 0xff) /* XXX too weak */
> goto nodevice;
>
>So who can tell me any details about that lovely weak check for R/W
>registers that appears to be failing for me? Simple things like:
> - Is this a valid register for a CDROM drive too? Ie, is this check
> tacitly assuming a hard disk beneath it?
> - Is it subject to timing problems?
> - It *claims* to be "too weak", however it appears to be too strong!
It's too weak for ST506...EIDE controllers with normal drives attached.
These all have read/write registers, so the inb() should return what
was written. That used to be tested for, but someone weakened the
test without documenting why. I don't know what happens for CDROM
drives.
The point of the test is to attempt to limit the damage if there is
a device other than an ST506...EIDE controller behind the port. It
is far too weak for that (if 0xff wasn't so magic, then the test
would have much less than a 1/256 chance of detecting conflicts).
Even if it tested for `== 0xa5', then any device with a read/write
port at the probed address would pass the test.
The test is very sloppy. It should do something like:
int
is_rw_port(port)
u_int_port;
{
u_char in1, in2, was;
DELAY(5);
was = inb(port);
DELAY(5);
outb(port, 0xa5);
DELAY(5);
(void)inb(0x20); /* attempt to eliminate bus echoes */
DELAY(5);
in1 = inb(port);
DELAY(5);
outb(port, 0x5a);
DELAY(5);
(void)inb(0x20);
DELAY(5);
in2 = inb(port);
DELAY(5);
outb(port, was);
DELAY(5);
return (in1 == 0xa5 && in2 == 0x5a);
}
...
if (!is_rw_port(du->dk_port + wd_cyl_lo))
goto nodevice;
Then the test would be stronger and your CDROM would be sure to fail :-).
Bruce
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