[9fans] Bizarre freeze

[9fans] Bizarre freeze

[sqweek]

 Ran into an interesting one setting up my cpu server. The following
reliably freezes the box:

# echo dma on >/dev/sdC0/ctl
# 9fat:

 Leaves the console totally unresponsive, no ^T^Tp, no ^P. rc -x on
9fat: gives "rfork e" as the final command.
 sdC0 is a cdrom. I can turn dma on for my sata disks (sdE0/sdF0) no problem.
-sqweek

Re: [9fans] Bizarre freeze

[Sape Mullender]

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I wonder why there is a command to turn on DMA on disks.  Could
it be that some disks don't work with DMA on?

	Sape

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From: sqweek <sqweek@gmail.com>
To: "Fans of the OS Plan 9 from Bell Labs" <9fans@9fans.net>
Subject: [9fans] Bizarre freeze
Date: Wed, 6 Aug 2008 02:01:17 +0800
Message-ID: <140e7ec30808051101n5df6bcdbx4ab515e643f5d9f@mail.gmail.com>

 Ran into an interesting one setting up my cpu server. The following
reliably freezes the box:

# echo dma on >/dev/sdC0/ctl
# 9fat:

 Leaves the console totally unresponsive, no ^T^Tp, no ^P. rc -x on
9fat: gives "rfork e" as the final command.
 sdC0 is a cdrom. I can turn dma on for my sata disks (sdE0/sdF0) no problem.
-sqweek

Re: [9fans] Bizarre freeze

[ron minnich]

On Tue, Aug 5, 2008 at 11:16 AM, Sape Mullender
<sape@plan9.bell-labs.com> wrote:
> I wonder why there is a command to turn on DMA on disks.  Could
> it be that some disks don't work with DMA on?

sure. it's PC hardware, so it's crap :-)
ron

Re: [9fans] Bizarre freeze

[erik quanstrom]

>> I wonder why there is a command to turn on DMA on disks.  Could
>> it be that some disks don't work with DMA on?
>
> sure. it's PC hardware, so it's crap :-)
> ron

the ata spec does not require dma for pata devices.
sata, first specified in ata 7, requires udma mode 5,
though sata:pata bridges routinely do not.

it's impossible to know without a bit more information,
but the dma command may be a red herring.

>  sdC0 is a cdrom. I can turn dma on for my sata disks (sdE0/sdF0) no problem.

can you access the cdrom at all?  i'm wondering if
you're accessing a ahci-capable chipset in ahci *and*
ata modes at the same time.

- erik

Re: [9fans] Bizarre freeze

[matt]

 > Could it be that some disks don't work with DMA on?
Not all compact flash supports DMA

Re: [9fans] Bizarre freeze

[erik quanstrom]

> > I wonder why there is a command to turn on DMA on disks.  Could
> > it be that some disks don't work with DMA on?

> sure. it's PC hardware, so it's crap :-)

unfortunately, this isn't a pc hardware thing.  pata
devices are not required to support dma.  ata 7, which
is the first to include sata support, requires udma mode
5 support.  (many sata:pata bridges violate this part of
the spec, e.g. sata doms.)

but i'm wondering if dma support isn't a red herring.

> sdC0 is a cdrom. I can turn dma on for my sata disks (sdE0/sdF0) no problem.

it could be that this is a chipset that advertizes sata and
pata pci devices concurrently, but only one may be accessed
by the operating system.  some ich 7 chipsets fall into this
category.

does this also hang your machine?

	echo probe|scuzz

if so, i would put some debugging in atapktio.
otherwise, it just might be a bug in sdata.c

- erik