Re: 2.6.19-rc3 system freezes when ripping with cdparanoia at ioctl(SG_IO)

["Monty Montgomery" <monty@xiph.org>]

On 11/9/06, Tejun Heo <htejun@gmail.com> wrote:

> drivers/scsi/sg.c interprets SG_DXFER_TO_FROM_DEV as read while
> block/scsi_ioctl.c interprets it as write.  I guess this is historic
> thing (scsi/sg.c updated but block/scsi_ioctl.c is forgotten).

Not historic; Jens accidentally implemented it backwards.  No one
noticed for a long time.  I submitted a patch for this a few months
ago.

> This works for most PATA ATAPI devices.  Most devices detect reversed
> transfer and terminate the command promptly.

No.  The rejection is *not* in hardware; it is in software.
block/scsi_ioctl.c, at least up to 2.6.16, rejected the TO_FROM_DEVICE
request when verifying the command for sanity after setting the
transfer direction incorrectly.  As far as the *device* can see,
TO_FROM_DEVICE and FROM_DEVICE are identical.  The difference only
applies inside the kernel mid-level driver where TO_FROM_DEVICE
prefills the transfer buffer as a way of working around having no
other detection path for short DMA transfers.

>  But this doesn't seem to
> be true for SATA device.

Then the driver is broken and needs to be fixed.  And I'll need to
find a workaround for broken kernels that doesn't cause a boom.

> Jens, I think we need to match block sg's behavior to SCSI's.  Monty,
> the timeout and hard lock up are due to hardware restrictions.

No., the kernel setting the transfer direction incorrectly.  I don't
set the transfer direction, the kernel does.

In your case, I pass in "SGIO_TO_FROM_DEVICE" and the kernel says
"that's a write".  The kernel is wrong.  It is a read.  The original
description of what TO_FROM_DEVICE is for is explicit on this point.

>  Kernel
> and libata can't do much about it.  So, please find other way to detect
> interface.

Just to be clear-- it is the kernel at fault here, and the kernel can
do something about it-- but only if the kernel gets fixed.  Also to be
clear, given this brokenness, yes I need to find another way.

Dammit, dammit, dammit, one step forward, two steps back :-(

Monty
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