alpha/75317: ATA DMA broken on PCalpha

[Sven Petai]

The following reply was made to PR alpha/75317; it has been noted by GNATS.

From: Sven Petai <hadara at bsd.ee>
To: freebsd-gnats-submit at freebsd.org
Cc:  
Subject: Re: alpha/75317: ATA DMA broken on PCalpha
Date: Wed, 22 Dec 2004 19:24:16 +0200

 I managed to put a wrong link in the original
 bugreport. The correct dmesg -v output is located at
 http://bsd.ee/~hadara/debug/pcalpha/pcalpha_panic_08.11.2004.txt
 
 
 I think I now understand what causes this bug,
 First of all there seems to be a slight bug in Alphas
 bus_dmamap_load(), which could be fixed with following
 patch:
 
 --- sys/alpha/alpha/busdma_machdep.c.orig       Wed Dec 22 17:11:27 2004
 +++ sys/alpha/alpha/busdma_machdep.c    Wed Dec 22 17:13:57 2004
 @@ -581,7 +581,8 @@
                 if (sg->ds_len == 0) {
                         sg->ds_addr = paddr + alpha_XXX_dmamap_or;
                         sg->ds_len = size;
 -               } else if (paddr == nextpaddr) {
 +               } else if (paddr == nextpaddr &&
 +            (sg->ds_len + size) <= dmat->maxsegsz) {
                         sg->ds_len += size;
                 } else {
                         /* Go to the next segment */
 
 
 without that we really could return larger chunks than
 specified by maxsegsz to bus_dma_tag_create function.
 
 anyway this still doesn't make things work correctly for me,
 because the real problem seems to be the Pyxis 
 page crossing bug. Basically it comes down to corrupting
 DMA transfers larger than 8k. It didn't cause problems before,
 since we never did larger than PAGE_SIZE transfers before 
 the ATA dma change mentioned in the original report. 
 There's a detection code for the buggy chip @
 src/sys/alpha/pci/cia.c
 but it's little too naive, since it assumes only DEC_ST550 can
 have it, in reality it seems to be used in some very early 
 revisions of 164LX(SX too?). But there doesn't seem to be a
 reliable way to detect if we have the faulty chip since
 it was worked around in later revisions by doing some changes
 elsewhere.
 
 One of the possible easy solutions would be to hack ata_dmaalloc() to
 use PAGE_SIZE as max segment size argument to the 
 bus_dma_tag_create function if machine has Pyxis chip at all,
 no matter if it's faulty or not.
 Would that be acceptable and if so what is the best way to propagate
 knowledge about existence of this chip from cia driver to ATA ?