cvs commit: src/sys/dev/usb ehci.c ehci_pci.c ehcivar.h

[Ian Dowse]

In message <20060131194244.GA75983 at saturn.kn-bremen.de>, Juergen Lock writes:
>>  Alright, looks like it was paging: (usb_block_allocmem ending up
>> calling contigmalloc...  which makes me wonder what would happen
>> there if someone had swap on a umass device?  swap here is on
>> ad4s2b which is on the sata card.)
>
>Ah, seems the actual problem is that it is sleeping although
>bus_dmamem_alloc is called with BUS_DMA_NOWAIT, and there even is
>a pr for that:
>	http://www.freebsd.org/cgi/query-pr.cgi?pr=78179
>The pr is still open so i guess there is no fix yet?

Ok, yes, this is a known problem that occurs if you access USB
devices for the first time when the physical memory is too fragmented
for a contiguous allocation to succeed immediately. A workaround
is to add more RAM or access the USB devices soon after booting.
Once the USB system has successfully allocated the memory it needs,
it will then work reliably.

In the case of USB, there is actually no need for it to perform
large contiguous allocations because the host controllers all support
some limited scatter-gather functionality so they can mostly access
the caller's memory buffer directly via bus_dmamap_load(). This is
something I implemented a year or to ago but I haven't got around
to finishing the last few details of the patch yet.

Ian