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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 13:04:03 +0100

On 13.02.2010, at 12:58, Aurelien Jarno wrote:

> On Sat, Feb 13, 2010 at 11:28:44AM +0100, Alexander Graf wrote:
>> 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.
> Exactly, that's the issue to fix here, make DBDMA work with CD-ROM so we
> can get rid of the cmd64x controller.

Speaking of which - in my PPC64 enabling series I use MacIO for all 4 IDE 
devices. At least with recent kernels, Linux just detects DMA being broken on 
the CD-ROM and doesn't use it.


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