Hi Mathias,
On 1/10/2023 12:03 PM, Wesley Cheng wrote:
Hi Mathias,
On 1/10/2023 11:47 AM, Mathias Nyman wrote:
On 9.1.2023 22.24, Wesley Cheng wrote:
Hi Mathias,
On 1/2/2023 8:38 AM, Mathias Nyman wrote:
On 29.12.2022 23.14, Wesley Cheng wrote:
Hi Mathias,
On 12/28/2022 7:47 AM, Mathias Nyman wrote:
On 24.12.2022 1.31, Wesley Cheng wrote:
Implement the XHCI operations for allocating and requesting for a
secondary
interrupter. The secondary interrupter can allow for events for a
particular endpoint to be routed to a separate event ring. The
event
routing is defined when submitting a transfer descriptor to the
USB HW.
There is a specific field which denotes which interrupter ring to
route the
event to when the transfer is completed.
An example use case, such as audio packet offloading can utilize
a separate
event ring, so that these events can be routed to a different
processor
within the system. The processor would be able to independently
submit
transfers and handle its completions without intervention from
the main
processor.
Adding support for more xHCI interrupters than just the primary
one make sense for
both the offloading and virtualization cases.
xHCI support for several interrupters was probably added to
support virtualization,
to hand over usb devices to virtual machines and give them their
own event ring and
MSI/MSI-X vector.
In this offloading case you probably want to avoid xHC interrupts
from this device
completely, making sure it doesn't wake up the main CPU
unnecessarily.
So is the idea here to let xhci driver set up the new interrupter,
its event ring,
and the endpoint transfer rings. Then pass the address of the
endpoint transfer rings
and the new event ring to the separate processor.
This separate processor then both polls the event ring for new
events, sets its dequeue
pointer, clears EHB bit, and queues new TRBs on the transfer ring.
so xhci driver does not handle any events for the audio part, and
no audio data URBs
are sent to usb core?
Your entire description is correct. To clarify, the interfaces
which are non-audio will still be handled by the main processor.
For example, a USB headset can have a HID interface as well for
volume control. The HID interface will still be handled by the
main processor, and events routed to the main event ring.
How about the control part?
Is the control endpoint for this device still handled normally by
usb core/xhci?
Control transfers are always handled on the main processor. Only
audio interface's endpoints.
Good to know, that means interrupter should be chosen per endpoint,
not per device.
For the xhci parts I think we should start start by adding generic
support for several
interrupters, then add parts needed for offloading.
I can split up the patchsets to add interrupters first, then adding
the offloading APIs in a separate patch.
I started looking at supporting secondary interrupters myself.
Let me work on that part a bit first. We have a bit different end
goals.
I want to handle interrupts from a secondary interrupter, while this
audio offload
really just wants to mask some interrupts.
I was looking at how we could possibly split up the XHCI secondary
interrupter, and offloading parts. Since the XHCI secondary
interrupter is a feature that is defined in the XHCI spec (and we
aren't doing anything outside of what is defined), I was thinking of
having a separate XHCI driver (ie xhci-sec.c/h) that can be used to
define all APIs related to setting up the event ring and ring
management. (interrupt support can be added here) This aligns a bit
with what Alan suggested, and removing the APIs in the USB HCD, since
this is XHCI specific stuff. (
https://lore.kernel.org/linux-usb/Y6zwZOquZOTZfnvP@xxxxxxxxxxxxxxxxxxx/
)
Already started working on the interrupter, that part fits well into
current driver.
Code (untested, will be randomly rebased etc) can be found in my
feature_interrupters branch:
git://git.kernel.org/pub/scm/linux/kernel/git/mnyman/xhci.git
feature_interrupters
https://git.kernel.org/pub/scm/linux/kernel/git/mnyman/xhci.git/log/?h=feature_interrupters
Oh perfect, let me take a look. Thanks for this!
I actually tried to see if I could get our audio offloading to work with
your current series. (I understand its still work in progress) I did
have to make some changes to expose the APIs to our class driver, but I
wanted to let you know about one of the issues I saw when developing my
implementation, because I am seeing the same behavior w/ yours. (and
there's a discrepancy w/ what's stated in the XHCI spec :))
So the reason why my initial submission did the event ring allocation
and set up before the run/stop bit was set, is that I found that when
writing to the ir_set->erst_base in this scenario (for the secondary
interrupter), it lead to a SMMU fault from the DWC3 controller. One
thing I noticed, was that the SMMU fault address was the lower 32 bits
of the segment table base address allocated. The XHCI driver utilizes
the xhci_write_64() api which first writes the lower 32 bits then the
upper 32 bits. The XHCI spec states that:
Table 5-41: Event Ring Segment Table Base Address Register Bit
Definitions (ERSTBA)
"Event Ring Segment Table Base Address Register – RW. Default = ‘0’.
This field defines the
high order bits of the start address of the Event Ring Segment Table.
Writing this register sets the Event Ring State Machine:EREP Advancement
to the Start state.
Refer to Figure 4-12 for more information.
**For Secondary Interrupters: This field may be modified at any time.**"
I'm not sure if this is an issue with the specific controller we're
using, so maybe I will wait until you can give this a try on your set
up. However, it doesn't seem to be true that we can write the ERSTBA
any time we want to. My assumption is that once I made the lower 32 bit
write, the controller attempted to enable the Event Ring State machine
(Figure 4-12), and this led to a SMMU fault, since the upper 64 bits
haven't been written. I also did some bit banging manually as well
(using devmem) and any time I write to the secondary ring ERSTBA
register it generates a fault. (before any offloading has started)
Thanks
Wesley Cheng
