ftp://ftp.cs.helsinki.fi/pub/Software/Linux/Kernel/testing/pre-patch-2.1.0.gz
(It's _currently_ has a md5sum of 92eb12e2275dd75fb14f6538d1a4e03d and is
28265 bytes long, but I might be updating it).
The new thing with the 2.1.x tree is that the kernel no longer uses the x86
segmentation to access user mode, but instead the kernel segment covers the
whole 4GB virtual address space, and thus accesses to user memory can be made
faster.
NOTE! When you access user memory you still have to use the "get_user()" etc
helper functions, exactly as before, because those can still do strange
things on other architectures. The new thing is just a x86 _implementation_
issue, not a change in the basic philosophy (you'll notice that most of the
changes are to x86-specific files).
The thing about the new memory management setup is that it gets rid of a lot
of extra fluff that isn't needed, and makes things potentially faster.
However, there is one downside to all this, and that is the fact that the
kernel address space is no longer identical to the hardware address space.
For example, to access hardware address 0xA0000, you can no longer just do
something like this:
value = (unsigned char *) 0xA0000;
but instead you have to use something like this:
value = readb(0xA0000);
Note that I called this a "downside", but in actual fact it's a good thing.
The reason that it's a good thing is that the drivers should have been using
readb() already - that's the portable interface, and that's how you have to
access PCI shared memory on alpha and PPC machines. In fact, most drivers
have already been converted to use this interface, but there are certainly
drivers out there that will break horribly by my pre-patch. That's why I'd
like driver authors (and technical people who want to test) to try out the
pre-patch.
Drivers that should work already (because they work on alpha's - the ones
marked with (*) have actually been tested by me on my PC with the pre-patch):
- VGA console (*)
- IDE harddisk and CD-ROM (*)
- normal serial lines (*)
- keyboard (*)
- 3c509 network card (*)
- floppy driver (*)
- PS/2 mouse
- de4x5 and ne2000 network cards
- original ncr scsi drivers
- BSD ncr SCSI driver (at least with IO-mapped accesses)
- Aha1740, BusLogic and QLogic ISP SCSI drivers
Other drivers are in the "may well work" category, but I cannot give any
guarantees. If some driver doesn't work, you're likely to find out sooner
rather than later (they generally break totally - it's not going to be
subtle).
I'd love to get reports on these patches, and would be even happier if you
can also send a patch that actually fixes any broken behaviour. Things
that can break are:
- accessing device memory though a pointer
FIX: use readb/writeb/memcpy_fromio/memcpy_toio instead of trying to
dereference the pointer directly.
- using virtual addresses for controller-initiated DMA
FIX: use the "virt_to_bus()" function to change a virtual address into
a "bus" address to give to controllers that do DMA into memory.
- memory re-mapping may be broken right now, so anything that needs
"vremap()" probably doesn't work. If you have a driver that breaks due
to this, please test changing the line (line 187 in mm/vmalloc.c)
set_pte(pte, mk_pte(offset, PAGE_KERNEL));
to do a
set_pte(pte, mk_pte(offset+PAGE_OFFSET, PAGE_KERNEL));
and see if that helps?
Comments? Success stories? Hate mail? Money?
Linus