[PATCH 135/379][SRU][OEM-5.6] bus: mhi: core: Handle write lock properly in mhi_pm_m0_transition

[You-Sheng Yang]

From: Hemant Kumar <hemantk at codeaurora.org>

BugLink: https://bugs.launchpad.net/bugs/1879633

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 at codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam at linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam at linaro.org>
Link: https://lore.kernel.org/r/20200521170249.21795-14-manivannan.sadhasivam@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit 3bc1a5f431618a656bbf674a4627ef4da3a0d893)
Signed-off-by: You-Sheng Yang <vicamo.yang at canonical.com>
---
 drivers/bus/mhi/core/pm.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/bus/mhi/core/pm.c b/drivers/bus/mhi/core/pm.c
index 345f197130b8..de5abb26bcae 100644
--- a/drivers/bus/mhi/core/pm.c
+++ b/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);
-- 
2.29.2