USB-audio isochronous Missed Service Errors on AMD Zen5 client (Fire Range) -- Data Fabric idle C-state? No OS-level knob found

["Gordon Chen" <chengordon326@gmail.com>]

Hi Mario, Shyam, Mathias,

I'm reporting what looks like an interaction between AMD client SoC idle
power management (Data Fabric / SOCCLK) and low-latency isochronous USB
DMA, on a Ryzen 9 9955HX3D (Fire Range, Zen5) laptop. It reproduces on any
AMD Zen4/5 client I or others have tried, with any USB audio interface;
there is an open community bug with several different-vendor devices [1].

I have a strong differential pointing below the OS, but no way to read DF
C-state residency on this platform to confirm -- hence this mail. Questions
are at the end.

Symptom
-------
USB audio playback has frequent audible clicks, and after ~10-30 min of
continuous playback an occasional full stall. Each click maps 1:1 (time-
aligned) to a short isochronous OUT URB; at the packet level these are
iso_frame_desc[i].status = -EXDEV (COMP_MISSED_SERVICE_ERROR) -- the xHCI
controller missed servicing a 125 us microframe. urb->error_count stays 0
and nothing reaches dmesg; you have to look at per-packet status or count
short OUT URBs. The stall is the same fault at its extreme: on an implicit-
feedback device, a whole errored capture URB starves the OUT ring until
re-plug.

What I tried (all ineffective except the last)
----------------------------------------------
  cpu_dma_latency = 0 (PM QoS, verified locked)        no change
  cpuidle: all C-states disabled                       no change
  cpufreq governor = performance                       no change
  amdgpu power_dpm_force_performance_level = high       no change
  stress-ng --cpu 16 (pure compute, little mem traffic) no change
  stress-ng --stream  (sustained memory bandwidth)     misses -> 0

So CPU residency/frequency is not the lever; sustained memory-controller
traffic is. Reading SoC clock DPM (amdgpu pp_dpm_socclk / pp_dpm_mclk):
idle and under --cpu it sits at socclk 400 MHz / mclk 1600 MHz; under
--stream it jumps to socclk 1200 / mclk 2800.

This points away from clock frequency as the lever: a single --stream
worker already pins socclk=1200 / mclk=2800, yet the misses persist; it
takes several workers of continuous traffic before they drop to zero. So
what tracks the fix is not the clock the SoC reaches, but the amount of
sustained memory-controller traffic. My working hypothesis is that the
Data Fabric idles into a low-power state between the sparse isochronous
transactions, and that the wake/traversal latency on the next microframe
can then exceed the 125 us deadline -- with only traffic dense enough to
keep the fabric out of that state avoiding it. I cannot confirm that from
the OS (I have no way to read DF C-state residency here), so I would
welcome a sanity check on the mechanism; the measurements above are what
I am confident of.

I could not reach this from the OS: profile_peak / force=high do not raise
socclk (ignored for an otherwise-idle iGPU), and a manual pp_dpm_socclk /
pp_dpm_mclk write is rejected with -EINVAL. The BIOS on this laptop exposes
no "DF C-States" / "Power Supply Idle Control" option.

Reproducer
----------
Any AMD Zen4/5 client + any USB audio interface, playing continuously:

  # count short OUT URBs (= missed-service events); nominal is the device's
  # full OUT packet size (768 bytes for the Flow 8 at 48 kHz / 4 ch)
  bpftrace -e 'kprobe:snd_complete_urb {
    $u = (struct urb *)arg0;
    if ((($u->pipe >> 7) & 1) == 0 && $u->actual_length < 768) { @miss++; }
  }'

  # while watching: cat /sys/class/drm/cardN/device/pp_dpm_socclk
  # baseline:           misses > 0,  socclk pinned at 400 MHz
  # with stress-ng --stream 4 running:  misses 0,  socclk 1200 MHz

Questions
---------
 1. Is this a known interaction between DF idle power state (DF C-states /
    SOCCLK gating) and latency-sensitive isochronous DMA on client SoCs --
    an erratum or documented behaviour?

 2. Is there a supported way to hold the Data Fabric out of its deep idle
    state (or set a fabric-active floor) while a latency-sensitive iso USB
    endpoint is streaming -- a kernel interface (amd_pmc / amd_node?), or a
    firmware/PMFW setting -- short of burning memory bandwidth?

 3. If not, what telemetry would help characterize this (amd_pmc STB dump?
    specific SMN / DF P-state or C-state registers? a debugfs)? I'm happy
    to build an instrumented kernel and test patches.

Related: the extreme stall case (whole-URB error on the implicitly-fed OUT
ring) is separately handled in snd-usb-audio by a Flow 8 quirk that emits
silence instead of hanging (commit a23812004228). That only papers over the
hang and does nothing about the clicks, which are the subject here.

System: Ryzen 9 9955HX3D (Fire Range, Zen5, 16C/32T), AMD xHCI, iGPU
smu_v13_0_0 (0x13c0); kernel 7.0.10 (CachyOS, close to mainline for
usb / xhci / amd power-management).

Thanks,
Gordon Chen

[1] https://github.com/CachyOS/linux-cachyos/issues/665