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Re: [Qemu-devel] qemu-system-ppc -m g3beige -hda is setting /dev/hdc on

From: Alexander Graf
Subject: Re: [Qemu-devel] qemu-system-ppc -m g3beige -hda is setting /dev/hdc on Linux.
Date: Sat, 13 Feb 2010 11:28:44 +0100

On 13.02.2010, at 09:02, Rob Landley wrote:

> The -hda, -hdb, -hdc, and -hdd command line options for g3beige don't match 
> the order the kernel assigns the drives.
> The reason is that the  Linux kernel always initializes the cmd646 driver 
> before the pmac driver, thus if there's a cmd646 it gets /dev/hda and 
> /dev/hdb, and the pmac gets /dev/hdc and /dev/hdb.
> If you only supply an -hda (and/or -hdb) with no -hdc or -hdd, then the 
> cmd646 
> driver never attaches to anything and only the pmac controller shows up, thus 
> -hda and -hdb set /dev/hda and /dev/hdb.  But if you specify a -hdc it shows 
> up as /dev/hda every time, and kicks the -hda entry to /dev/hdc.
> Note that neither the kernel's CONFIG_BLK_DEV_IDE_PMAC_ATA100FIRST nor 
> CONFIG_IDEPCI_PCIBUS_ORDER made any difference, because those affect multiple 
> devices handled by the same driver, and this is a static driver 
> initialization 
> order issue.  When you statically link in both drivers, cmd64x always probes 
> before pmac due to the above hardwired device order in the kernel, 100% 
> reliable and deterministic.  It's hardwired, and you have to patch the kernel 
> to change it. 
> Here's a patch to the Linux kernel that changes the device probe order so the 
> kernel behaves like g3beige is expecting it to:
> --- a/drivers/ide/Makefile
> +++ b/drivers/ide/Makefile
> @@ -39,6 +39,7 @@
> obj-$(CONFIG_BLK_DEV_AMD74XX)          += amd74xx.o
> obj-$(CONFIG_BLK_DEV_ATIIXP)           += atiixp.o
> obj-$(CONFIG_BLK_DEV_CELLEB)           += scc_pata.o
> +obj-$(CONFIG_BLK_DEV_IDE_PMAC)         += pmac.o
> obj-$(CONFIG_BLK_DEV_CMD64X)           += cmd64x.o
> obj-$(CONFIG_BLK_DEV_CS5520)           += cs5520.o
> obj-$(CONFIG_BLK_DEV_CS5530)           += cs5530.o
> @@ -76,8 +77,6 @@
> obj-$(CONFIG_BLK_DEV_CMD640)           += cmd640.o
> -obj-$(CONFIG_BLK_DEV_IDE_PMAC)         += pmac.o
> -
> obj-$(CONFIG_IDE_H8300)                        += ide-h8300.o
> obj-$(CONFIG_IDE_GENERIC)              += ide-generic.o
> The problem is, the kernel guys will never take that patch upstream because 
> what they're currently doing isn't actually wrong.  Their behavior is 
> consistent, the kernel's been probing the same devices in the same order 
> since 
> the 90's, and they don't really care what order things go in.
> The problem is that the association between qemu's command line arguments and 
> the devices they refer to is somewhat arbitrary.  On the other targets I've 
> used (arm, mips, x86, and so on), the device QEMU initializes in response to 
> "-hda" is the one the Linux kernel makes /dev/hda (or /dev/sda), and the one 
> it intializes in response to "-hdc" is the one Linux makes /dev/hdc.  But in 
> this case, they don't match up, and that's screwing up my same init/build 
> script that works fine on all the other tarets.
> Here's a patch to QEMU that makes those arguments intialize the devices the 
> kernel expects them to.  This doesn't change where any of the hardware is on 
> the board, just which command line arguments associate with which drives:

This is wrong. On my OpenSUSE 11.1 guest the devices come up in correct order. 
They also do so on Aurelien's Debian images (IIRC). I guess it mostly works 
fine when using modules instead of compiled in drivers.

Please find a real G3 beige and see what's different on it. I'd bet the real 
difference is that all 4 devices are attached to MacIO. But from what I 
remember DBDMA with cd-roms wasn't considered stable, hence the use of cmd64x 
on the second channel.


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