linux-kernel - [PATCH 5/5] bus: mhi: host: Remove redundant dma

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Date:   Mon,  2 May 2022 16:11:44 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     mhi@...ts.linux.dev
Cc:     linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        quic_hemantk@...cinc.com, quic_bbhatt@...cinc.com,
        loic.poulain@...aro.org,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Subject: [PATCH 5/5] bus: mhi: host: Remove redundant dma_wmb() before ctx wp update

The endpoint device will only read the context wp when the host rings
the doorbell. And moreover the doorbell write is using writel(). This
guarantess that the prior writes will be completed before ringing
doorbell.

So there is no need of an additional dma_wmb() to order the coherent
memory writes w.r.t each other. Even if the writes gets reordered, it
won't affect the endpoint device.

Cc: Loic Poulain <loic.poulain@...aro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
---
 drivers/bus/mhi/host/main.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
index 966ffc2458b9..6706a82d3aa8 100644
--- a/drivers/bus/mhi/host/main.c
+++ b/drivers/bus/mhi/host/main.c
@@ -138,11 +138,6 @@ void mhi_ring_chan_db(struct mhi_controller *mhi_cntrl,
 
 	db = ring->iommu_base + (ring->wp - ring->base);
 
-	/*
-	 * Writes to the new ring element must be visible to the hardware
-	 * before letting h/w know there is new element to fetch.
-	 */
-	dma_wmb();
 	*ring->ctxt_wp = cpu_to_le64(db);
 
 	mhi_chan->db_cfg.ring_db(mhi_cntrl, &mhi_chan->db_cfg,
-- 
2.25.1