Re: I did a test on a DEC AlphaStation 600 and the SCSI driver seems borked

[Magnus Lindholm <linmag7%gmail.com@localhost>]

> Hm, I added support for the monster window to NetBSD/alpha in:
>
> $NetBSD: tsp_dma.c,v 1.17 2021/05/27 22:11:31 thorpej Exp $
>
> (Only for Tsunami, etc. systems)
>

This is good news! I'll have a look at the code again, I forgot which version
I tested this on. As I understand it, in order to actually use the
monster window
it first must be enabled byt setting  PCTL<MWIN>. Then in order for it to be
actually used one must enable by generating physical addresses
that have  TSUNAMI_DAC_OFFSET (1UL << 40) (bit 40 set).



>
> This suggests to me that I should probably change the tsp_dma code, then, to enable and advertise the monster direct-mapped window only on systems that actually could benefit from it (i.e. systems with > 2GB RAM).
>

I guess the 2GB mapping comes from how Linux has organized the memory on Alpha.
It is possible that NetBSD is doing something different here. Either
way, it is unlikely
that using the monster window will have any benefit on systems with
less than 2GB RAM.
My issues with tsunami based systems and the ISP1040 card was that men
using DAC/monster
windows (i.e addressing bit 40 was set) I would see sporadic data
corruptions.  Always occuring
in chunks of 64-bytes, so some DMA transfers got through, but every
now and then data would
get corrupted.


From the Tsunami/Typhoon manual:

There can be four standard DMA windows and one DMA monster window. The
DMA monster
window is enabled by PCTL<MWIN>. If enabled, this window lies from
100.0000.0000 to
100.FFFF.FFFF, which requires a dual-address cycle (DAC) access from
the PCI bus.
This window maps to system memory as defined in Section 10.1.4

> -- thorpej
>