Re: Sym53c8xx tape corruption squashed! (was: Re: SCSI Tape corruption - update)

["Gérard Roudier" <groudier@free.fr>]

On Fri, 28 Dec 2001, Geert Uytterhoeven wrote:

> On Wed, 5 Dec 2001, Geert Uytterhoeven wrote:
> > On Fri, 2 Nov 2001, [ISO-8859-1] Gérard Roudier wrote:
> > > On Thu, 1 Nov 2001, Geert Uytterhoeven wrote:
> > > As driver sym-2 is planned to replace sym53c8xx in the future, it would be
> > > interesting to give it a try on your hardware. There are some source
> > > available from ftp.tux.org, but I can provide you with a flat patch
> > > against the stock kernel version you want. You may let me know.
> >
> > I tried sym-2 (2.4.17-pre2) and it didn't show up the problem, which is good!
> >
> > More news from the old driver:
> >
> >     1.5c            OK
> >     1.5d            OK
> >     1.5e            page fault in interrupt handler 0xa53c0c68
> >     1.5f            lock up
> >     1.5pre-g1       lock up
> >     1.5pre-g2       lock up
> >     1.5pre-g3       corruption
> >     1.5g            corruption
> >
> > So it happened somewhere in between 1.5d and 1.5pre-g3. I'll see whether I can
> > get any of the intermediates to run...
>
> I made all intermediate versions to work.
>
> The problem is introduced in 1.5pre-g2 by the following change:
>
> diff -urN callisto-1.5g-pre2a/sym53c8xx.c callisto-1.5g-pre2+/sym53c8xx.c
> --- callisto-1.5g-pre2a/sym53c8xx.c	Fri Dec 28 21:12:30 2001
> +++ callisto-1.5g-pre2+/sym53c8xx.c	Fri Dec 28 20:11:10 2001
> @@ -11981,7 +11981,7 @@
>  	**    (latency timer >= burst length + 6, we add 10 to be quite sure)
>  	*/
>
> -	if ((pci_fix_up & 4) && chip->burst_max) {
> +	if (chip->burst_max && (latency_timer == 0 || (pci_fix_up & 4))) {
>  		uchar lt = (1 << chip->burst_max) + 6 + 10;
>  		if (latency_timer < lt) {
>  			printk(NAME53C8XX
>
> This change causes the PCI latency timer to be changed from 0 to 80.
>
> The sym-2 driver has a define for modifying the PCI latency timer
> (SYM_SETUP_PCI_FIX_UP), but it is never used, so I see no corruption.

By default sym-2 use value 3 for the pci_fix_up (cache line size + memory
write and invalidate). The latency timer fix-up has been removed, since it
is rather up to the generic PCI driver to tune latency timers.

> Is this a hardware bug in my SCSI host adapter (53c875 rev 04) or my host
> bridge (VLSI VAS96011/12 Golden Gate II for PPC), or a software bug in the
> driver (wrong burst_max)?

Great bug hunting!

It is about certainly not a software bug in the driver. Any latency timer
value should not give any trouble if hardware was flawless. Just the PCI
performances could be affected.

Anyway, value 0 looks way stupid for devices capable of bursting more than
1 data phase, thus the improvement above. :)

> To recapitulate, the bug causes error bursts of (almost always) 32 bytes long.
> The incorrect bytes are always a copy of previous data, at a fixed offset (10
> kiB on my (now dead) DDS-1 tape drive, 32 kiB on my Plexwriter).

Unfortunately, I haven't the errata listing for teh 53c875 rev 4. I have
the DEL for 875 rev. 3 and for 876 rev. 5.

If we assume that rev 4 hasn't more bugs than rev 3, then you may try to
disable MEMORY WRITE and INVALIDATE (and not tell the driver to fix this
up) but allow the driver to fix the bogus zero latency timer. The 875 rev
3 may, under certain conditions, execute unaligned PCI MEMORY WRITE and
INVALIDATE transactions. Note that this may explain data corruptions
occurring for SCSI READ commands not WRITE commands. No other items can
explain, on paper, data corruptions of the form you describe due to 875
chip misbehaviour.

Btw, latency timer zero should not change the likelyhood of this item.
This let me think that the host bridge is likely to be the culprit.

> Gr{oetje,eeting}s,

Gr{oudier,eat bug hunting, indeed}. :)