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The choice of U1 and U2 timeouts for USB 3.0 Link Power Management (LPM)
is highly host controller specific. Here are a few examples of why it's
host specific:
1. Setting the U1/U2 timeout too short may cause the link to go into
U1/U2 in between service intervals, which some hosts may tolerate,
and some may not.
2. The host controller has to modify its bus schedule in order to take
into account the Maximum Exit Latency (MEL) to bring all the links
from the host to the device into U0. If the MEL is too big, and it
takes too long to bring the links into an active state, the host
controller may not be able to service periodic endpoints in time.
3. Host controllers may also have scheduling limitations that force
them to disable U1 or U2 if a USB device is behind too many tiers of
hubs.
We could take an educated guess at what U1/U2 timeouts may work for a
particular host controller. However, that would result in a binary
search on every new configuration or alt setting installation, with
multiple failed Evaluate Context commands. Worse, the host may blindly
accept the timeouts and just fail to update its schedule for U1/U2 exit
latencies, which could result in randomly delayed periodic transfers.
Since we don't want to cause jitter in periodic transfers, or delay
config/alt setting changes too much, lay down a framework that xHCI
vendors can extend in order to add their own U1/U2 timeout policies.
To extend the framework, they will need to:
- Modify the PCI init code to add a new xhci->quirk for their host, and
set the XHCI_LPM_SUPPORT quirk flag.
- Add their own vendor-specific hooks, like the ones that will be added
in xhci_call_host_update_timeout_for_endpoint() and
xhci_check_tier_policy()
- Make the LPM enable/disable methods call those functions based on the
xhci->quirk for their host.
An example will be provided for the Intel xHCI host controller in the
next patch.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
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| core | ||
| dwc3 | ||
| early | ||
| gadget | ||
| host | ||
| image | ||
| misc | ||
| mon | ||
| musb | ||
| otg | ||
| phy | ||
| renesas_usbhs | ||
| serial | ||
| storage | ||
| wusbcore | ||
| Kconfig | ||
| Makefile | ||
| README | ||
| usb-common.c | ||
| usb-skeleton.c | ||
To understand all the Linux-USB framework, you'll use these resources:
* This source code. This is necessarily an evolving work, and
includes kerneldoc that should help you get a current overview.
("make pdfdocs", and then look at "usb.pdf" for host side and
"gadget.pdf" for peripheral side.) Also, Documentation/usb has
more information.
* The USB 2.0 specification (from www.usb.org), with supplements
such as those for USB OTG and the various device classes.
The USB specification has a good overview chapter, and USB
peripherals conform to the widely known "Chapter 9".
* Chip specifications for USB controllers. Examples include
host controllers (on PCs, servers, and more); peripheral
controllers (in devices with Linux firmware, like printers or
cell phones); and hard-wired peripherals like Ethernet adapters.
* Specifications for other protocols implemented by USB peripheral
functions. Some are vendor-specific; others are vendor-neutral
but just standardized outside of the www.usb.org team.
Here is a list of what each subdirectory here is, and what is contained in
them.
core/ - This is for the core USB host code, including the
usbfs files and the hub class driver ("khubd").
host/ - This is for USB host controller drivers. This
includes UHCI, OHCI, EHCI, and others that might
be used with more specialized "embedded" systems.
gadget/ - This is for USB peripheral controller drivers and
the various gadget drivers which talk to them.
Individual USB driver directories. A new driver should be added to the
first subdirectory in the list below that it fits into.
image/ - This is for still image drivers, like scanners or
digital cameras.
../input/ - This is for any driver that uses the input subsystem,
like keyboard, mice, touchscreens, tablets, etc.
../media/ - This is for multimedia drivers, like video cameras,
radios, and any other drivers that talk to the v4l
subsystem.
../net/ - This is for network drivers.
serial/ - This is for USB to serial drivers.
storage/ - This is for USB mass-storage drivers.
class/ - This is for all USB device drivers that do not fit
into any of the above categories, and work for a range
of USB Class specified devices.
misc/ - This is for all USB device drivers that do not fit
into any of the above categories.