mailbox: pcc: Fix the possible race in updation of chan_in_use flag

The function mbox_chan_received_data() calls the Rx callback of the
mailbox client driver. The callback might set chan_in_use flag from
pcc_send_data(). This flag's status determines whether the PCC channel
is in use.

However, there is a potential race condition where chan_in_use is
updated incorrectly due to concurrency between the interrupt handler
(pcc_mbox_irq()) and the command sender(pcc_send_data()).

The 'chan_in_use' flag of a channel is set to true after sending a
command. And the flag of the new command may be cleared erroneous by
the interrupt handler afer mbox_chan_received_data() returns,

As a result, the interrupt being level triggered can't be cleared in
pcc_mbox_irq() and it will be disabled after the number of handled times
exceeds the specified value. The error log is as follows:

  |  kunpeng_hccs HISI04B2:00: PCC command executed timeout!
  |  kunpeng_hccs HISI04B2:00: get port link status info failed, ret = -110
  |  irq 13: nobody cared (try booting with the "irqpoll" option)
  |  Call trace:
  |   dump_backtrace+0x0/0x210
  |   show_stack+0x1c/0x2c
  |   dump_stack+0xec/0x130
  |   __report_bad_irq+0x50/0x190
  |   note_interrupt+0x1e4/0x260
  |   handle_irq_event+0x144/0x17c
  |   handle_fasteoi_irq+0xd0/0x240
  |   __handle_domain_irq+0x80/0xf0
  |   gic_handle_irq+0x74/0x2d0
  |   el1_irq+0xbc/0x140
  |   mnt_clone_write+0x0/0x70
  |   file_update_time+0xcc/0x160
  |   fault_dirty_shared_page+0xe8/0x150
  |   do_shared_fault+0x80/0x1d0
  |   do_fault+0x118/0x1a4
  |   handle_pte_fault+0x154/0x230
  |   __handle_mm_fault+0x1ac/0x390
  |   handle_mm_fault+0xf0/0x250
  |   do_page_fault+0x184/0x454
  |   do_translation_fault+0xac/0xd4
  |   do_mem_abort+0x44/0xb4
  |   el0_da+0x40/0x74
  |   el0_sync_handler+0x60/0xb4
  |   el0_sync+0x168/0x180
  |  handlers:
  |   pcc_mbox_irq
  |  Disabling IRQ #13

To solve this issue, pcc_mbox_irq() must clear 'chan_in_use' flag before
the call to mbox_chan_received_data().

Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Tested-by: Robbie King <robbiek@xsightlabs.com>
Signed-off-by: Huisong Li <lihuisong@huawei.com>
(sudeep.holla: Minor updates to the subject, commit message and comment)
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

Author: LiHuiSong1

drivers/mailbox/pcc.c

@@ -333,10 +333,16 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
 	if (pcc_chan_reg_read_modify_write(&pchan->plat_irq_ack))
 		return IRQ_NONE;
 
+	/*
+	 * Clear this flag after updating interrupt ack register and just
+	 * before mbox_chan_received_data() which might call pcc_send_data()
+	 * where the flag is set again to start new transfer. This is
+	 * required to avoid any possible race in updatation of this flag.
+	 */
+	pchan->chan_in_use = false;
 	mbox_chan_received_data(chan, NULL);
 
 	check_and_ack(pchan, chan);
-	pchan->chan_in_use = false;
 
 	return IRQ_HANDLED;
 }