ATA errors.

[Matthew Dillon]

:I found that there's a place where ATA driver silently falls back to
:BIOSPIO mode even when bootverbose is true. In ad_start(), if the
:call to ata_dmaalloc fails, adp->device->mode is set to ATA_PIO, but
:there's no ata_prtdev() around there, that's why we don't see any
:`fallback' messages. I'm not sure why ata_dmaalloc() failed.
:Anyway, attached is a patch to add ata_prtdev(), and add a knob
:hw.ata.panic_on_dma_failure(default: 0).

    Ah, Excellent.  It's because it is using M_NOWAIT and the allocation
    can fail if kernel_map is locked.  This is just bad algorithm design
    because allocations with M_NOWAIT can fail even on FreeBSD-4.x boxes.

    I will encapsulate the dmatab freeing code and make it unconditional.

					-Matt