[PATCH v4 0/2] usb: musb: gadget: don't flush a waiting RX packet

[Elvira Khabirova <lineprinter0@gmail.com>]

Since commit 3fc137386c46 ("usb: musb: Fix hardware lockup on first Rx
endpoint request"), musb_ep_restart() flushes the RX FIFO instead of
calling rxstate().

When using some functions with musb, for example, f_mass_storage
or f_acm, this code path will be used mid-communication, and it will
cause data loss.

Patch 2 fixes this in musb_ep_restart() and schedules a bh work item
instead of flushing and losing the data in the RX FIFO.

Patch 1 modifies the semantics of musb_ep.busy from a flag to a counter.
If usb_ep_dequeue() runs at the same time as another musb_g_giveback(),
`busy` could end up with a false 1; this would stall an endpoint.
I added this patch in the series as prep for patch 2, because patch 2
would otherwise add one more case where such a race is possible --
a musb_g_giveback() of a work item with a musb_g_giveback() of nuke().

v3 -> v4:
- Move musb_gadget_enable_rx_kicks() in musb_resume() to be
  after musb_enable_interrupts() and under musb->lock (addresses
  sashiko's "premature and unlocked rx_kick queueing in musb_resume()")

v2 -> v3:
- Added patch 1: `busy` is a counter now (addresses sashiko's
  "overlapping musb_g_giveback() calls wedge ep->busy" and
  the nuke() part of "race and ordering bug in musb_gadget_disable()
  for rx_kick")
- Schedule a kick on every OUT ep when resuming to avoid stalls when
  the USB session is preserved across suspend/resume
  (addresses sashiko's "lost rx_kick across system suspend and resume")

v1 -> v2:
- musb_g_rx_kick() does not re-queue itself when `busy` is set.
  Instead, musb_g_giveback() re-queues it when `rx_kick_deferred`.
  This new flag is set in three cases:
  1) `busy` is set inside musb_g_rx_kick()
  2) musb_gadget_queue() adds an RX request to an empty request queue
     and `busy` is set
  3) `busy` is set in the beginning of musb_g_rx() (addresses sashiko's
     "RX endpoint lockup after giveback in musb_g_rx_kick()",
     "softirq busy-loop and livelock in musb_g_rx_kick()", and
     "race and ordering bug in musb_gadget_disable() for rx_kick")
- musb_g_rx_kick checks musb->is_runtime_suspended now and exits early
  if the controller is suspended, and the work items are disabled
  during system suspend (addresses sashiko's
  "unushed rx_kick work in musb_suspend() accesses disabled MMIO")
- This time tested also on a Pine A64+ (Allwinner A64),
  which has both MUSB and 4 cores.

v3: https://lore.kernel.org/linux-usb/20261107173601.63095-1-lineprinter0@gmail.com/
v2: https://lore.kernel.org/linux-usb/20261107105505.6554-1-lineprinter0@gmail.com/
v1: https://lore.kernel.org/linux-usb/20261027202744.79556-1-lineprinter0@gmail.com/

Elvira Khabirova (2):
  usb: musb: gadget: count givebacks in busy
  usb: musb: gadget: don't flush a waiting RX packet when queueing a
    request

 drivers/usb/musb/musb_core.c   |   3 +
 drivers/usb/musb/musb_gadget.c | 128 ++++++++++++++++++++++++++++++---
 drivers/usb/musb/musb_gadget.h |  11 ++-
 3 files changed, 133 insertions(+), 9 deletions(-)


base-commit: c339f4010dea8b0d754f84ace70cd1b089878ef8
-- 
2.53.0