[PATCH] improved TT scheduling for EHCI

This updates the EHCI driver by adding an improved scheduler for the
transaction translators, found in USB 2.0 hubs and used for low and
full speed devices.

 - adds periodic_tt_usecs() and some helper functions, which does
   the same thing that "periodic_usecs" does, except on the other
   side of the TT, i.e.  it calculates the low/fullspeed bandwidth
   usage instead of highspeed.

 - adds a tt_available() function which is the new implementation
   of what tt_no_collision() does ... while tt_no_collision() ensures
   that each TT handles only 1 periodic transfer at a time (a very
   pessimistic approach) this version instead tracks bandwidth and
   allows each TT to handle as many transfers as will fit on each TT's
   downstream bus (closer to best-case).

The new scheduler is selected by a config option, marked as EXPERIMENTAL
so it can be tested (and more broadly reviewed) for a while until it
seems safe to remove the original scheduler.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 19 insertions, 1 deletions

diff --git a/drivers/usb/host/Kconfig b/drivers/usb/host/Kconfig
index e27b79a3c05f..c060eb9b3b19 100644
--- a/drivers/usb/host/Kconfig
+++ b/drivers/usb/host/Kconfig
@@ -47,7 +47,25 @@ config USB_EHCI_ROOT_HUB_TT
           controller is needed.  It's safe to say "y" even if your
           controller doesn't support this feature.
 
-          This supports the EHCI implementation from TransDimension Inc.
+          This supports the EHCI implementation that's originally
+          from ARC, and has since changed hands a few times.
+
+config USB_EHCI_TT_NEWSCHED
+        bool "Improved Transaction Translator scheduling (EXPERIMENTAL)"
+        depends on USB_EHCI_HCD && EXPERIMENTAL
+        ---help---
+          This changes the periodic scheduling code to fill more of the low
+          and full speed bandwidth available from the Transaction Translator
+          (TT) in USB 2.0 hubs.  Without this, only one transfer will be
+          issued in each microframe, significantly reducing the number of
+          periodic low/fullspeed transfers possible.
+
+          If you have multiple periodic low/fullspeed devices connected to a
+          highspeed USB hub which is connected to a highspeed USB Host
+          Controller, and some of those devices will not work correctly
+          (possibly due to "ENOSPC" or "-28" errors), say Y.
+
+          If unsure, say N.
 
 config USB_ISP116X_HCD
         tristate "ISP116X HCD support"