reason for VIA performance drop since 2.4.2-ac21

reason for VIA performance drop since 2.4.2-ac21

[Jacek Popławski]

I have motherboard with VIA MVP3 chipset. 

I noticed big video slowdown since 2.4.2-ac21. Watching "divx" by avifile with
new kernels is impossible, becouse very bad performance. Now, after few hours -
I found the reason, and I don't understand it.

It has nothing to do with mtrr or K6.  In file arch/i386/kernel/pci-pc.c there
is a pci_fixup_via691_2 function.  It appeared in 2.4.2-ac21. And it works for
my chipset - VIA_82C598. When I put "return" in body of this function,
recompile and start kernel 2.4.4 - "x11perf -putimage100" shows that video
works fast again.

I see two possibilities:

1) this is just a debug code, and kernels >2.4.2-ac20 shouldn't be used by VIA
MVP3 owners
2) this code fix crash possibility, and all kernels without it (including
2.2.19) are buggy with VIA MVP3

What is true, and is it safe to skip that function if I need 2.4 and fast video?

Re: reason for VIA performance drop since 2.4.2-ac21

[Alan Cox]

> It has nothing to do with mtrr or K6.  In file arch/i386/kernel/pci-pc.c there
> is a pci_fixup_via691_2 function.  It appeared in 2.4.2-ac21. And it works for
> my chipset - VIA_82C598. When I put "return" in body of this function,
> recompile and start kernel 2.4.4 - "x11perf -putimage100" shows that video
> works fast again.

My log says that this was a change pulled in from Linus tree. I don't know who
put it there or why

> 1) this is just a debug code, and kernels >2.4.2-ac20 shouldn't be used by VIA
> MVP3 owners
> 2) this code fix crash possibility, and all kernels without it (including
> 2.2.19) are buggy with VIA MVP3

Im not aware of any write posting bugs on the MVP3 but I dont follow the
chipset in detail.