Re: [PATCH] fixup for pci config_access on alchemy au1x000

[Sergei Shtylyov <sshtylyov@ru.mvista.com>]

Hello.

Alexander Bigga wrote:

> I've encountered a serious problem with PCI config space access on Au1x000 
> platforms with recent 2.6.x-kernel. With 2.4.31 the same hardware works fine. 
> So I was looking for the differences:

> Symptoms:
> - no PCI-device is seen on bootup though two or three cards are present
> - lspci output is empty
> - OR: lspci shows 20 times the same device
> (- OR: in some slot-configurations it worked anyhow)

> System(s): 
> 1. platform with Au1500 and three PCI-devices (actually a mycable XXS1500 
>     with backplane for three PCI-devices)
> 2. platform with Au1550 and two PCI-devices (custom board)

> Debugging:
> I digged down to the config_access() of the au1xxx-processors in 
> arch/mips/pci/ops-au1000.c and switched on DEBUG.

> The code of config_access() seems to be almost the same as of the 
> 2.4.x-kernel. But the "pci_cfg_vm->addr" returned by get_vm_area(0x2000, 0) 
> once on booting is different. That's of course not forbidden. But the 
> alignment seems to be wrong. In my case, I received:

> 2.4.31: pci_cfg_vm->addr = c0000000
> 2.6.18-rc5: pci_cfg_vm->addr = c0101000

> To make it short: With 2.6.x it fails on the first config-access with:
> "PCI ERR detected: status 83a00356".

> Fixup:
> My fix is now, to use the VM_IOREMAP-flag in the get_vm_area call. This flag 
> seems to be introduced in mm/vmalloc.c a long time ago (in 2.6.7-bk13, I 
> found in gitweb). 
> Now, the returned address is pci_cfg_vm->addr = c0104000 and everything works 
> fine.

> What do you think about my fixup-patch? 
> Nobody's using the get_vm_area()-call without any flag ("0"). Was it only 
> forgotten in the  arch/mips/pci/ops-au1000.c?

> Or am I completely wrong?

    You're correct -- this code was only working by some chance. Once you get 
a virtual address not aligned to 8K, it breaks completely since the pages 
can't constitute a valid pair for wired entry anymore.
    Actually, in 2.4 the situation seems to be even worse as get_vm_area() 
there has no provision for the address alignment at all!

> Best regard,

> Alexander

WBR, Sergei