#15569: usb_disk stalls & unmounts when large I/O transfers are not queued
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Reporter: X512 | Owner: nobody
Type: bug | Status: new
Priority: normal | Milestone: Unscheduled
Component: Drivers/Disk/USB | Version: R1/Development
Resolution: | Keywords:
Blocked By: | Blocking:
Has a Patch: 0 | Platform: All
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Comment (by pulkomandy):
As to why the usb_disk device stalls:
https://git.haiku-os.org/haiku/tree/src/add-
ons/kernel/drivers/disk/usb/usb_disk/usb_disk.cpp#n695
Here you can see that USB mass storage has a 2-stage process. First it
sends a command, and then it read or writes the data. What happens here is
that the first step succeeds, but the second one fails. This leaves the
device waiting for the data transfer forever.
We can do better error handling to recover from the stall, I guess, rather
than unmounting the disk. However, it's hard to know the state of the
device then. Did it "gracefully" fail just that transfer? Or did it crash
and also lost all the data that was still in its cache?
I don't see why it should be usb_disk responsibility to split the transfer
in smaller chunks. It can't know the limitations of the underlying levels,
and should let those decide how to handle things. At the hardware level
there does not appear to be any constraint, so, if we know we will be
reading a very large chunk, I don't see why we shouldn't do that in a
single go.
There can be two reasons to limit the size, and both are out of control of
usb_disk:
- Limitations of the underlying USB stack, if we can't remove them, there
should be a way to ask for the max allowed size, I would not hardcode it
in usb_disk,
- Reducing latency (as large transfers will keep the device busy for some
time): this should be done in io_scheduler, the usb_disk driver is not the
right place to take such decisions.
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Ticket URL: <https://dev.haiku-os.org/ticket/15569#comment:11>
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