#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):
Yes, what FreeBSD does makes more sense, I think. The "cam" layer in
FreeBSD is basically an abstracted storage device system which is more or
less based on what SCSI provides in terms of commands. Underlying drivers
implement the actual communication with devices. We have a somewhat
similar scsi kernel module but currently usb_disk does not use it.
It is of course not possible (nor desirable) to transparently split the
transfers at xhci level beyond its maximum size. But, from usb_disk point
of view, the most efficient thing to do is "as large as possible" (if we
consider bandwidth as the thing we want to optimize). So, ideally, there
should be a way to know what's the limit of the underlying usb module, and
usb_disk would split transfers to that size.
as for the splitting, the simplest thing to do is probably to cap the
block count in usb_disk_needs_partial_buffer. The read() call of usb_disk
will then do a partial read (it adjusts its returned length) and the upper
layer should then call it again as needed (assuming it handles partial
reads properly).
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Ticket URL: <https://dev.haiku-os.org/ticket/15569#comment:15>
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