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

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 <[email protected]>
>> ---
>> 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