[PATCH v2 0/4] 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 a separate segment_pool
associated to each sideband instance.  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.  Which pool is selected is all based on if the sideband path
is being enabled, and if so, memory can be allocated from that pool,
which expects to be owned in conjunction with the audio DSP.  This
concept allows to keep the same model existing in xHCI, where multiple
4k segments can reside on the same page, which reduces potentially over
allocating based on the page size.

Likewise this mechanism also allows for the offload client driver to
determine which SID is associated to the segment_pool if it decides to
map outside of the Linux subsystem.  The new ring allocation flow for
sideband/offload clients will be as follows:

qc_usb_audio_offload_probe()
  ├─ segment_pool = dma_pool_create(...)
  ▼
xhci_sideband_register(intf, XHCI_SIDEBAND_VENDOR, segment_pool, notify_client)
  │  sb->segment_pool = segment_pool
  ▼
uadev[card_num].sb = sb

handle_uaudio_stream_req()
  ▼
enable_audio_stream(subs, ..., pcm_card_num)
  ├─ data_ep = uaudio_find_host_endpoint(subs, subs->data_endpoint)
  ├─ xhci_sideband_add_endpoint(sb, data_ep) ← ep->sideband = sb; sb->eps[ep_index] = ep
  ├─ snd_usb_endpoint_prepare(chip, sync_endpoint) ─┐
  ├─ snd_usb_endpoint_prepare(chip, data_endpoint)  ├─→ xhci_check_bandwidth()
  │                                                 ▼
  │            xhci_endpoint_init(xhci, virt_dev, ep, ...)
  │            pool = sideband ? sideband->segment_pool : xhci->segment_pool
  │            new_ring = xhci_ring_alloc_from_pool(..., pool, ...)
  │            ▼
  │            xhci_ring_alloc_from_pool(..., pool, flags)
  │            ring->segment_pool = pool
  │            ▼
  │            xhci_alloc_segments_for_ring(xhci, ring, flags)
  │            xhci_segment_alloc(xhci, ring->segment_pool, max_packet, num, flags)
  │            ▼
  │            xhci_segment_alloc(xhci, pool, max_packet, num, flags)
  │            seg->trbs = dma_pool_zalloc(pool, flags, &dma)
  ▼
xhci_sideband_get_endpoint_buffer(sb, data_ep) → xhci_ring_to_sgtable()

qc_usb_audio_offload_disconnect() / unreg_xhci:
  ├─ segment_pool = sb->segment_pool
  ├─ xhci_sideband_unregister(sb)
  ▼
  dma_pool_destroy(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.

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>
---
Changes in v2:
- Moved from using alignment_req to having the offload driver maintain
  its own segment pool.
- Fixed OOB condition seen in xhci_ring_to_sgtable() and will capture
  page offsets properly.
- Addressed inter-patch build failures.
- Link to v1: https://patch.msgid.link/20260824-16k_offload_v1_b4-v1-0-49a6be60ca30@xxxxxxxxxxxxxxxx

To: Mathias Nyman <mathias.nyman@xxxxxxxxx>
To: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
To: Jaroslav Kysela <perex@xxxxxxxx>
To: Takashi Iwai <tiwai@xxxxxxxx>
Cc: linux-usb@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-sound@xxxxxxxxxxxxxxx

---
Wesley Cheng (4):
      xhci: sideband: fix ring sg table for sub-page TRB segments
      usb: xhci: sideband: allocate sideband ring segments from a dedicated pool
      ALSA: usb-audio: qcom: tag sideband endpoints before ring allocation
      ALSA: usb-audio: qcom: fix xfer ring IOMMU unmap on 16K+ page kernels

 drivers/usb/host/xhci-mem.c       |  64 ++++++++++++------
 drivers/usb/host/xhci-sideband.c  |  22 +++++-
 drivers/usb/host/xhci.h           |  14 ++--
 include/linux/usb/xhci-sideband.h |  22 +++++-
 sound/usb/qcom/qc_audio_offload.c | 137 +++++++++++++++++++++++++++++++-------
 5 files changed, 202 insertions(+), 57 deletions(-)
---
base-commit: e1e6e541c5c9cf548e9fdc35fc26808c82074440
change-id: 20260824-16k_offload_v1_b4-3d1460405774

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