Reserving an ATA interface

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi Bart, Alan !

I've been fighting with a problem with both 2.4 and 2.5 for which I
would have a workaround, but I'd like to have your point of
view on it first as this code is rather messy.

So the problem is all about deciding if an hwif slot is 'owned' by
a driver, even after ide_unregister is called, after probe, etc...
and problems with the way the current scheme works (and differences
between the way PCI interfaces "pick" free slots vs. ide_register_hw
does)

So, let's first explain the various cases I have to deal with in the
context of ide/ppc/pmac.c.

1 - Normal case: interface is non hotswap, drive is present at startup,
things just work normally

2 - Empty interface: at boot, en empty interface (no drive) is found.
This happens a lot since Apple's ASIC often have at least one spare
ATA/33 interface that isn't wired to anything. This hwif is filled
up for ide/ppc/pmac (MMIO iops, chipset = ide_pmac, etc...).
hwif->present is cleared by the common probe code

3 - Hotswap interface: This one may or may not have devices plugged
at boot (laptop media bay). The hwif is filled with MMIO iops etc...
but since it may not have the CD drive plugged at boot, hwif->present
may be cleared by the common probe code.

So now, the problems:

 - In both case 2 and 3, hwif->present may be cleared. That means
that something calling ide_register_hw() (like ide-cs for example)
will eventually pick up that hwif slot and try to use it. However
the hwif beeing filled for ide/ppc/pmac.c (MMIO iops among other)
it will usually not work and just crash, since ide_register_hw()
will _not_ reinit the fields. Also, in the case of the hotswap
interface (3), It's simply not acceptable for the slot to be re-used
(while it is for case 2) since you don't want the CD-ROM location
to change, and because ide-pmac will have internally setup a data
structure for this hwif that is tied to a given index that was
obtained at boot.

So that leads to 2 actual needs: One, to deal with case 2 & re-use
of the hwif by ide-cs (among others) is to have ide_register_hw
actually clear the interface calling init_hwif_data() at some
point when picking a "new" slot. The other one is to be able to
"mark" an interface as "held" by the driver (hotswap bay that
don't want to change numbering).

The simplest solution I have in mind is to add an hwif flag,
called "hold" (or whatever better name you find). Drivers like
ide/ppc/pmac.c would set this flag for the "hotswap" media bay
interface, and not for others.

The only change to the core code would then be for ide_register_hw
to 'skip' those when searching for an available slot, and to call
init_hwif_data when (!hwif->present && !hwif->hold) to handle case 2
where the iops & other hwif fields (mmio among others) need to be
reset to initial/legacy state.

That would allow "empty" fixed pmac interfaces to be released for
use by ide-cs, while the hotswap media bay one stay reserved.

That would give us a patch like that:

===== include/linux/ide.h 1.47 vs edited =====
--- 1.47/include/linux/ide.h	Sun Apr 27 00:11:51 2003
+++ edited/include/linux/ide.h	Sat May  3 11:30:34 2003
@@ -1022,6 +1022,7 @@
 
 	unsigned	noprobe    : 1;	/* don't probe for this interface */
 	unsigned	present    : 1;	/* this interface exists */
+	unsigned	hold       : 1; /* this interface is always present */
 	unsigned	serialized : 1;	/* serialized all channel operation */
 	unsigned	sharing_irq: 1;	/* 1 = sharing irq with another hwif */
 	unsigned	reset      : 1;	/* reset after probe */
===== drivers/ide/ide.c 1.45 vs edited =====
--- 1.45/drivers/ide/ide.c	Sun Apr 27 00:11:50 2003
+++ edited/drivers/ide/ide.c	Sat May  3 11:32:35 2003
@@ -920,8 +920,8 @@
 		}
 		for (index = 0; index < MAX_HWIFS; ++index) {
 			hwif = &ide_hwifs[index];
-			if ((!hwif->present && !hwif->mate && !initializing) ||
-			    (!hwif->hw.io_ports[IDE_DATA_OFFSET] && initializing))
+			if (!hwif->hold && ((!hwif->present && !hwif->mate && !initializing) ||
+			    (!hwif->hw.io_ports[IDE_DATA_OFFSET] && initializing)))
 				goto found;
 		}
 		for (index = 0; index < MAX_HWIFS; index++)
@@ -931,6 +931,8 @@
 found:
 	if (hwif->present)
 		ide_unregister(index);
+	else if (!hwif->hold)
+		init_hwif_data(index);
 	if (hwif->present)
 		return -1;
 	memcpy(&hwif->hw, hw, sizeof(*hw));