bus: mhi: core: Handle write lock properly in mhi_pm_m0_transition

Take write lock only to protect db_mode member of mhi channel.
This allows rest of the mhi channels to just take read lock which
fine grains the locking. It prevents channel readers to starve if
they try to enter critical section after a writer.

Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20200521170249.21795-14-manivannan.sadhasivam@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/bus/mhi/core/pm.c

@@ -288,14 +288,18 @@ int mhi_pm_m0_transition(struct mhi_controller *mhi_cntrl)
 	for (i = 0; i < mhi_cntrl->max_chan; i++, mhi_chan++) {
 		struct mhi_ring *tre_ring = &mhi_chan->tre_ring;
 
-		write_lock_irq(&mhi_chan->lock);
-		if (mhi_chan->db_cfg.reset_req)
+		if (mhi_chan->db_cfg.reset_req) {
+			write_lock_irq(&mhi_chan->lock);
 			mhi_chan->db_cfg.db_mode = true;
+			write_unlock_irq(&mhi_chan->lock);
+		}
+
+		read_lock_irq(&mhi_chan->lock);
 
 		/* Only ring DB if ring is not empty */
 		if (tre_ring->base && tre_ring->wp  != tre_ring->rp)
 			mhi_ring_chan_db(mhi_cntrl, mhi_chan);
-		write_unlock_irq(&mhi_chan->lock);
+		read_unlock_irq(&mhi_chan->lock);
 	}
 
 	mhi_cntrl->wake_put(mhi_cntrl, false);