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

To: Alexander Bigga <ab@mycable.de>
Subject: Re: [PATCH] fixup for pci config_access on alchemy au1x000
From: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Date: Tue, 29 Aug 2006 21:43:25 +0400
Cc: linux-mips@linux-mips.org
In-reply-to: <200608291648.35250.ab@mycable.de>
Organization: MontaVista Software Inc.
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <200608291648.35250.ab@mycable.de>
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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

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