I was in need of a 3.3V/dual-voltage-capable PCI sound-card and, as
luck would have it, the only card of that sort in the local computer
store was one that _still_ has a DMA engine that cannot even DMA to
all 32 bits (it's limited to 28 bits).  Hard to believe, but true (the
card in question is a "SoundBlaster Live! 24-bit" with a CA0106 chip;
stay away from that one if you can...).

Anyhow, since I don't like it when PCI cards don't work in my machine,
I created the attached patch which adds a new machine-vector to enable
support of such broken cards.

With the patch applied, you can either configure the kernel for
"HP-zx1/sx1000+swiotlb" or configure for "generic" and boot with
option "machvec=hpzx1_swiotlb" to enable support for broken PCI
devices.

The patch works as follows: the new machvec implements a I/O MMU which
will use the hardware I/O MMU whenever possible but fall back on the
software I/O TLB when encountering a device that can't be supported by
the hardware I/O MMU.  Fortunately, we don't have to mess with
MAX_DMA_ADDRESS or create a new zone: the software I/O TLB allocates
its memory as low as possible and early in the boot-process, so on any
machine with low memory, we're pretty much guaranteed that we'll get a
reasonable amount of low memory, which is all we need to properly support
broken PCI cards.

Note that I made a small change to swiotlb.c: I added a
swiotlb_init_with_default_size() function to let the new I/O MMU
initialize the software I/O TLB with less than 64MB (which is way too
much for the limited uses we'll see for the broken PCI devices; for
example, the CA0106 chip allocates only coherent buffers of about
128KB).

The patch has been tested on a zx1 machine in the generic, hpzx1, and
hpzx1_swiotlb configuration.

Thanks to Alex Williamson for the suggestion of doing this via a
completely separate machvec.
Signed-off-by: David Mosberger-Tang <davidm@hpl.hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>