For periodic endpoints, we must let the xHCI hardware know the maximum
payload an endpoint can transfer in one service interval. The xHCI
specification refers to this as the Maximum Endpoint Service Interval Time
Payload (Max ESIT Payload). This is used by the hardware for bandwidth
management and scheduling of packets.
For SuperSpeed endpoints, the maximum is calculated by multiplying the max
packet size by the number of bursts and the number of opportunities to
transfer within a service interval (the Mult field of the SuperSpeed
Endpoint companion descriptor). Devices advertise this in the
wBytesPerInterval field of their SuperSpeed Endpoint Companion Descriptor.
For high speed devices, this is taken by multiplying the max packet size by the
"number of additional transaction opportunities per microframe" (the high
bits of the wMaxPacketSize field in the endpoint descriptor).
For FS/LS devices, this is just the max packet size.
The other thing we must set in the endpoint context is the Average TRB
Length. This is supposed to be the average of the total bytes in the
transfer descriptor (TD), divided by the number of transfer request blocks
(TRBs) it takes to describe the TD. This gives the host controller an
indication of whether the driver will be enqueuing a scatter gather list
with many entries comprised of small buffers, or one contiguous buffer.
It also takes into account the number of extra TRBs you need for every TD.
This includes No-op TRBs and Link TRBs used to link ring segments
together. Some drivers may choose to chain an Event Data TRB on the end
of every TD, thus increasing the average number of TRBs per TD. The Linux
xHCI driver does not use Event Data TRBs.
In theory, if there was an API to allow drivers to state what their
bandwidth requirements are, we could set this field accurately. For now,
we set it to the same number as the Max ESIT payload.
The Average TRB Length should also be set for bulk and control endpoints,
but I have no idea how to guess what it should be.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
9238f25d5d