The busy loop in rpmh_rsc_send_data() is written with the assumption
that the udelay will be preempted by the tcs_tx_done() irq handler when
the TCS slots are all full. This doesn't hold true when the calling
thread is an irqthread and the tcs_tx_done() irq is also an irqthread.
That's because kernel irqthreads are SCHED_FIFO and thus need to
voluntarily give up priority by calling into the scheduler so that other
threads can run.

I see RCU stalls when I boot with irqthreads on the kernel commandline
because the modem remoteproc driver is trying to send an rpmh async
message from an irqthread that needs to give up the CPU for the rpmh
irqthread to run and clear out tcs slots.

 rcu: INFO: rcu_preempt self-detected stall on CPU
 rcu:     0-....: (1 GPs behind) idle=402/1/0x4000000000000002 softirq=2108/2109 fqs=4920
  (t=21016 jiffies g=2933 q=590)
 Task dump for CPU 0:
 irq/11-smp2p    R  running task        0   148      2 0x00000028
 Call trace:
  dump_backtrace+0x0/0x154
  show_stack+0x20/0x2c
  sched_show_task+0xfc/0x108
  dump_cpu_task+0x44/0x50
  rcu_dump_cpu_stacks+0xa4/0xf8
  rcu_sched_clock_irq+0x7dc/0xaa8
  update_process_times+0x30/0x54
  tick_sched_handle+0x50/0x64
  tick_sched_timer+0x4c/0x8c
  __hrtimer_run_queues+0x21c/0x36c
  hrtimer_interrupt+0xf0/0x22c
  arch_timer_handler_phys+0x40/0x50
  handle_percpu_devid_irq+0x114/0x25c
  __handle_domain_irq+0x84/0xc4
  gic_handle_irq+0xd0/0x178
  el1_irq+0xbc/0x180
  save_return_addr+0x18/0x28
  return_address+0x54/0x88
  preempt_count_sub+0x40/0x88
  _raw_spin_unlock_irqrestore+0x4c/0x6c
  ___ratelimit+0xd0/0x128
  rpmh_rsc_send_data+0x24c/0x378
  __rpmh_write+0x1b0/0x208
  rpmh_write_async+0x90/0xbc
  rpmhpd_send_corner+0x60/0x8c
  rpmhpd_aggregate_corner+0x8c/0x124
  rpmhpd_set_performance_state+0x8c/0xbc
  _genpd_set_performance_state+0xdc/0x1b8
  dev_pm_genpd_set_performance_state+0xb8/0xf8
  q6v5_pds_disable+0x34/0x60 [qcom_q6v5_mss]
  qcom_msa_handover+0x38/0x44 [qcom_q6v5_mss]
  q6v5_handover_interrupt+0x24/0x3c [qcom_q6v5]
  handle_nested_irq+0xd0/0x138
  qcom_smp2p_intr+0x188/0x200
  irq_thread_fn+0x2c/0x70
  irq_thread+0xfc/0x14c
  kthread+0x11c/0x12c
  ret_from_fork+0x10/0x18

This busy loop naturally lends itself to using a wait queue so that each
thread that tries to send a message will sleep waiting on the waitqueue
and only be woken up when a free slot is available. This should make
things more predictable too because the scheduler will be able to sleep
tasks that are waiting on a free tcs instead of the busy loop we
currently have today.
Reviewed-by: Maulik Shah <mkshah@codeaurora.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Maulik Shah <mkshah@codeaurora.org>
Cc: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200724211711.810009-1-sboyd@kernel.orgSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>