CVS commit: src/sys/dev/usb

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Wed Feb 12 16:00:34 UTC 2020

Modified Files:
        src/sys/dev/usb: ehci.c

Log Message:
Fix steady state of timeouts in ehci.

This is complicated because:

1. There are three ways that an xfer can be completed:
   (a) hardware interrupt completes xfer
   (b) software decision aborts xfer with USBD_CANCELLED
   (c) timeout aborts xfer with USBD_TIMEOUT

2. The timeout abort can't be done in callout because ehci_sync_hc,
   called unconditionally by ehci_abort_xfer to wait until the device
   has finished using any references to the xfer, may sleep.  So we
   have to schedule a callout that, when run, will schedule a usb_task.

3. The hardware completion interrupt can't sleep waiting for a callout
   or task to finish -- can't use callout_halt or usb_rem_task_wait.
   So the callout and usb_task must be able to run _after_ the hardware
   completion interrupt, and recognize that they're late to the party.

   (Note, though, that usbd_free_xfer does wait for the callout and
   task to complete, so there's no danger they may use themselves after
   free.)

4. The xfer may resubmitted -- and the timeout may be rescheduled --
   immediately after the hardware completion interrupt, _while_ the
   callout and/or usb_task may still be scheduled.  Specifically, we
   may have the following sequence of events:
   (a) hardware completion interrupt
   (b) callout or usb_task fires
   (c) driver resubmits xfer
   (d) callout or usb_task acquires lock and looks around dazed and
       bewildered at the firehose of events like reading the news in 2019

The mechanism for sorting this out is that we have two bits of state:

- xfer->ux_timeout_set informs the driver, when submitting an xfer and
  setting up its timeout, whether either the callout or usb_task is
  already scheduled or not.

- xfer->ux_timeout_reset informs the callout or usb_task whether it
  should reschedule the callout, because the xfer got resubmitted, or
  not.


To generate a diff of this commit:
cvs rdiff -u -r1.269 -r1.270 src/sys/dev/usb/ehci.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.