Re: [PATCH 0/2] Add larger page size support for USB audio offload path

[Wesley Cheng]

On 8/25/2026 4:09 AM, Takashi Iwai wrote:
> On Tue, 25 Aug 2026 04:06:54 +0200,
> Wesley Cheng wrote:
> > On some environments, 16kB pages can be enabled from the Linux subsystem,
> > which manages the IOMMU mappings for the audio DSP within the system.  In
> > the current design, the following assumptions break when 16k pages are
> > utilized:
> >    1. xHCI ring size is equal to PAGE_SIZE
> >    2. Ring addresses start at the beginning of a page
> >
> > When the USB offload driver maps the rings (w/ the audio DSP SID), it is
> > set with a 16k granular, which is a problem, as several xHCI rings could
> > exist on the same page.  This is because the rings are currently allocated
> > from the segment_pool.  Hence, potentially mapping non USB audio related
> > rings into the region accessible by the audio DSP.
> >
> > To mitigate this, this series introduces the alignment_req parameter.
> > Before the USB audio offload path is enabled, the USB audio data
> > streams/endpoint are not active.  Only when the class driver issues a
> > usb_set_interface() call (done from snd_usb_endpoint_prepare()), will the
> > xHCI allocate the transfer ring resources.  By setting the alignment_req
> > beforehand, when allocating the ring segment, it can fulfill the audio DSP
> > alignment requirements by allocating DMA-able memory on the fly (based on
> > what is being requested) versus fetching it from the segment pool.
> > Likewise, keep track of if memory was dynamically allocated to handle the
> > free path properly.  The function call flow will now look like the
> > following:
> >
> > handle_uaudio_stream_req()
> >    │
> >    ▼
> > enable_audio_stream(subs, ..., pcm_card_num)
> >    │
> >    ├─ xhci_sideband_add_endpoint(sb, data_ep, PAGE_SIZE)
> >    │    │  alignment_req == PAGE_SIZE
> >    │    ▼
> >    │  sb->alignment_req = alignment_req
> >    │
> >    ├─ snd_usb_endpoint_prepare(chip, data_endpoint)
> >    │     → xhci_check_bandwidth() → xhci_endpoint_init())
> >    │    ▼
> >    xhci_endpoint_init(..., ep_index, ...)
> >    │    if (sideband && sideband->alignment_req)
> >    │        new_ring = xhci_ring_alloc(xhci, 2, ring_type, max_packet,
> >    │                                    sideband->alignment_req, mem_flags)
> >    │    ▼
> >    xhci_ring_alloc(..., alignment_req, ...)
> >    │    ring->alignment_req = alignment_req
> >    │    ▼
> >    xhci_alloc_segments_for_ring(xhci, ring, flags)
> >    │    xhci_segment_alloc(xhci, ..., ring->alignment_req, flags)
> >    │    ▼
> >    xhci_segment_alloc(..., alignment_req, flags)
> >         if (alignment_req > TRB_SEGMENT_SIZE)
> >             seg->trbs = dma_alloc_coherent(dev, alignment_req, &dma, flags)
> >         else
> >             seg->trbs = dma_pool_zalloc(xhci->segment_pool, ...)
> >
> > Similar logic is added for the secondary interrupter path as well.  The USB
> > offload class driver calls xhci_sideband_create_interrupter(), which will
> > be responsible for allocating the secondary event ring.  The same
> > alignment_req parameter is passed, and during xHCI event ring creation, the
> > same set of APIs are utilized, so the runtime memory allocation is already
> > handled.
> >
> > This was confirmed to work on the SM8350 MTP platform, with the
> > CONFIG_ARM64_16K_PAGES config enabled, alongside tinyaudio binaries:
> >
> > tinymix -D 0 set 513 1  (Enables USB_RX multimedia#1 path)
> > tinyplay -D 0 -d 0....  (Routes PCM data to ASoC platform sound card)
> >
> > Signed-off-by: Wesley Cheng <wesley.cheng@xxxxxxxxxxxxxxxx>
> > ---
> > Wesley Cheng (2):
> >        xhci: sideband: support page-aligned ring segment allocation
> >        ALSA: usb-audio: qcom: request page-aligned xHCI ring buffers
>
> I guess your first patch alone breaks the build, and this is bad for
> bisection.  When you change the API, the callers should be addressed
> in the same commit altogether in order to keep the stuff working
> during the transition.

Hi Takashi,

Understood, I will figure out how to adjust these patches so that 
incremental builds don't break on the next revision.

Thanks
Wesley Cheng

> thanks,
>
> Takashi