linux-kernel - Re: [RFC PATCH 07/14] usb: host: xhci: Add XHCI secondary interrupter support

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Date:   Mon, 2 Jan 2023 18:38:56 +0200
From:   Mathias Nyman <mathias.nyman@...ux.intel.com>
To:     Wesley Cheng <quic_wcheng@...cinc.com>,
        srinivas.kandagatla@...aro.org, mathias.nyman@...el.com,
        perex@...ex.cz, broonie@...nel.org, lgirdwood@...il.com,
        andersson@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        gregkh@...uxfoundation.org, Thinh.Nguyen@...opsys.com,
        bgoswami@...cinc.com, tiwai@...e.com, robh+dt@...nel.org,
        agross@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        alsa-devel@...a-project.org, devicetree@...r.kernel.org,
        linux-usb@...r.kernel.org, quic_jackp@...cinc.com,
        quic_plai@...cinc.com
Subject: Re: [RFC PATCH 07/14] usb: host: xhci: Add XHCI secondary interrupter
 support

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.

Thanks
Mathias