Reading DVD video broken in recent current

[Steve O'Hara-Smith]

	Hi,

	Now using an up to date preview I get various DMA errors when
trying to watch or otherwise read an encrypted DVD using either acd or cd
devices. Errors include:

Feb 27 12:33:54 df1 kernel: ata1: FAILURE - non aligned DMA transfer
attempted Feb 27 12:33:54 df1 kernel: acd1: setting up DMA failed

and

Feb 27 11:56:24 df1 kernel: acd1: setting up DMA failed
Feb 27 11:56:24 df1 kernel: ata1: FAILURE - oversized DMA transfer attempt
131072 > 65536

	Strangely mounting the DVD (either via cd or acd devices) works
fine and I can read the encrypted vobs with no trouble (sum works fine for
example). I can also play DVDs by using natacontrol to turn off DMA but it
eats CPU horribly.

	It seems that something in libdvdcss is triggering the problem
which is odd as that is purely userland code - the only clue I can see is
that it seems to use readv a lot so perhaps there is some path by which
readv is doing bad things with DMA ?

	I'll investigate more as time permits - if I can find the code for
readv that is.

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