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

[Wesley Cheng]

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

 drivers/usb/host/xhci-mem.c       | 70 ++++++++++++++++++++--------
 drivers/usb/host/xhci-sideband.c  |  9 ++--
 drivers/usb/host/xhci.h           | 11 ++++-
 include/linux/usb/xhci-sideband.h |  7 ++-
 sound/usb/qcom/qc_audio_offload.c | 96 +++++++++++++++++++++++++++++++++------
 5 files changed, 152 insertions(+), 41 deletions(-)
---
base-commit: e1e6e541c5c9cf548e9fdc35fc26808c82074440
change-id: 20260824-16k_offload_v1_b4-3d1460405774

Best regards,
--  
Wesley Cheng <wesley.cheng@xxxxxxxxxxxxxxxx>