st.ko with usb-storage problems

[Matthew Dharm <mdharm-scsi@one-eyed-alien.net>]

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(NOTE: Cross-posted to several lists.  Watch your replies.)

For a while now in 2.5 and 2.6, I've been getting reports of an
incompatibility between st.ko and the usb-storage driver.  I've finally
managed to figure out the problem; it only affects certain hardware, and
only on certain operations.

USB is a packet-oriented protocol.  Important to the protocol is the
concept of a packet being full or not -- in a given transfer, all packets
should be full except the last one.  Since usb-storage maps the SCSI
command's scatter-gather list for direct I/O, each scatter-gather segment
(except for the last one) must be a multiple of the packet size.  That
value can vary, but is always a power of 2 (I've personally seen up to
512 bytes).

When st.ko goes to do a read, it will create a scatter-gather list with
(what looks like) a mapping of the user-space buffer.  This often results
in not-so-nicely sized segments, which are often a multiple of 8, but not
64, 128, or 512 (which are the common USB packet sizes).  This causes a
problem for the USB host-controller hardware, and the transfer fails.

Why this doesn't happen on a write is something of a mystery to me, but it
doesn't seem to happen.

If the module flag 'try_direct_io' is set to 0 (not the default), then
st.ko doesn't generate scatter-gather lists at all, and all is well.

It's not immediately clear to me how to fix this.  First, I need to figure
out why 2.4 kernels don't show this problem, I think.  Help from people who
understand st.ko would be appreciated.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@one-eyed-alien.net 
Maintainer, Linux USB Mass Storage Driver

THEY CASTRATED MY QUAKE BITS! I WANT THEM BACK!!!!
					-- Greg
User Friendly, 3/27/1998

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