Messages in this thread | | | Date | Thu, 9 Nov 2006 15:09:22 -0500 | From | "Monty Montgomery" <> | Subject | Re: 2.6.19-rc3 system freezes when ripping with cdparanoia at ioctl(SG_IO) |
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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 :-(
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