Workaround for AMD762MPX "mouse" bug
Quoted 28 lines · Andi Kleen
AMD has published a new errata sheet for the AMD762, which describes the root cause of the infamous "AMD 762 unstable when no PS/2 mouse connected" bug. The reason is that without a PS/2 mouse the BIOS doesn't put a data page in front of the VGA buffer at 640K. When the kernel puts a page cache page there and does busmaster IO with it then the automatic PCI prefetch from the chipset can hit the VGA buffer and that may cause a hang.
The workaround is to reserve the page directly before 640K if it wasn't already reserved by the BIOS.
The bug only occurs in newer revisions (B0,B1)
The workaround here is somewhat hackish. We can only reserve the page in early boot, but at that time there is no easy way to check for the AMD762's PCI-ID because the PCI subsystem hasn't been initialized yet.
This patch checks later during the pci quirks pass instead and then tells the user to pass a kernel option - "vgaguard" - in case of instability. This is not ideal, but probably preferable than to connect PS/2 mouses to all boxes in a colocated rack. Another way would be to always reserve that page, but I didn't feel like punishing everybody just for a hardware bug in a single chipset.
Quoted 1 line
Patch for 2.5.53. Please consider applying.
Some suggestions :
- do not tell the user to use the "vgaguard" option if he is already
using it
- change to more informative text :
old :
I/O APIC: AMD762 Errata #56 may be present.
In case of instability boot with "vgaguard" or connect a PS/2 mouse. new:
I/O APIC: AMD762 Errata #56 may be present.
In case of instability boot with the "vgaguard" kernel boot option or connect a PS/2 mouse and reboot.
Just connecting a PS/2 mouse on a running system does not help, right ? :-)
- maybe rename the option to "amd762vgaguard" ?
- also write some docs and put a link to it in the kernel message ?
For now this would be enough :
I/O APIC: AMD762 Errata #56 may be present.
In case of instability boot with the "vgaguard" kernel boot option or connect a PS/2 mouse and reboot.
See http://www.uwsg.indiana.edu/hypermail/linux/kernel/0212.3/0043.html
Regards,
David Balazic
Quoted 2 lines · David Balazic
Just connecting a PS/2 mouse on a running system does not help, right ? :-)
It has to occur at boot. The fix proposed is crap though. Its perfectly possible to reserve the page at boot up time and give it back later if the errata is not found and it isnt in the EBDA.