bus: mhi: host: Fix race between channel preparation and M0 event

There is a race condition where mhi_prepare_channel() updates the
read and write pointers as the base address and in parallel, if
an M0 transition occurs, the tasklet goes ahead and rings
doorbells for all channels with a delta in TRE rings assuming
they are already enabled. This causes a null pointer access. Fix
it by adding a channel enabled check before ringing channel
doorbells.

Cc: stable@vger.kernel.org # 5.19
Fixes: a6e2e352 "bus: mhi: core: Add support for PM state transitions"
Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://lore.kernel.org/r/1665889532-13634-1-git-send-email-quic_qianyu@quicinc.com
[mani: CCed stable list]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

drivers/bus/mhi/host/pm.c

@@ -301,7 +301,8 @@ int mhi_pm_m0_transition(struct mhi_controller *mhi_cntrl)
 		read_lock_irq(&mhi_chan->lock);
 
 		/* Only ring DB if ring is not empty */
-		if (tre_ring->base && tre_ring->wp  != tre_ring->rp)
+		if (tre_ring->base && tre_ring->wp  != tre_ring->rp &&
+		    mhi_chan->ch_state == MHI_CH_STATE_ENABLED)
 			mhi_ring_chan_db(mhi_cntrl, mhi_chan);
 		read_unlock_irq(&mhi_chan->lock);
 	}