linux-kernel - Re: [PATCH] i2c: i2c-qcom-geni: Fix DMA transfer race

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Date:   Tue, 21 Jul 2020 00:07:55 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>,
        Wolfram Sang <wsa@...-dreams.de>
Cc:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Akash Asthana <akashast@...eaurora.org>,
        Douglas Anderson <dianders@...omium.org>,
        Alok Chauhan <alokc@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Wolfram Sang <wsa@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i2c: i2c-qcom-geni: Fix DMA transfer race

Quoting Stephen Boyd (2020-07-20 22:59:14)
> 
> I worry that we also need a dmb() here to make sure the dma buffer is
> properly mapped before this write to the device is attempted. But it may
> only matter to be before the I2C_READ.
> 

I'm suggesting this patch instead where we make geni_se_setup_m_cmd()
use a writel() so that it has the proper barrier semantics to wait for
the other memory writes that happened in program order before this point
to complete before the device is kicked to do a read or a write.

----8<----
diff --git a/drivers/i2c/busses/i2c-qcom-geni.c b/drivers/i2c/busses/i2c-qcom-geni.c
index 18d1e4fd4cf3..7f130829bf01 100644
--- a/drivers/i2c/busses/i2c-qcom-geni.c
+++ b/drivers/i2c/busses/i2c-qcom-geni.c
@@ -367,7 +367,6 @@ static int geni_i2c_rx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
 		geni_se_select_mode(se, GENI_SE_FIFO);
 
 	writel_relaxed(len, se->base + SE_I2C_RX_TRANS_LEN);
-	geni_se_setup_m_cmd(se, I2C_READ, m_param);
 
 	if (dma_buf && geni_se_rx_dma_prep(se, dma_buf, len, &rx_dma)) {
 		geni_se_select_mode(se, GENI_SE_FIFO);
@@ -375,6 +374,8 @@ static int geni_i2c_rx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
 		dma_buf = NULL;
 	}
 
+	geni_se_setup_m_cmd(se, I2C_READ, m_param);
+
 	time_left = wait_for_completion_timeout(&gi2c->done, XFER_TIMEOUT);
 	if (!time_left)
 		geni_i2c_abort_xfer(gi2c);
@@ -408,7 +409,6 @@ static int geni_i2c_tx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
 		geni_se_select_mode(se, GENI_SE_FIFO);
 
 	writel_relaxed(len, se->base + SE_I2C_TX_TRANS_LEN);
-	geni_se_setup_m_cmd(se, I2C_WRITE, m_param);
 
 	if (dma_buf && geni_se_tx_dma_prep(se, dma_buf, len, &tx_dma)) {
 		geni_se_select_mode(se, GENI_SE_FIFO);
@@ -416,6 +416,8 @@ static int geni_i2c_tx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
 		dma_buf = NULL;
 	}
 
+	geni_se_setup_m_cmd(se, I2C_WRITE, m_param);
+
 	if (!dma_buf) /* Get FIFO IRQ */
 		writel_relaxed(1, se->base + SE_GENI_TX_WATERMARK_REG);
 
diff --git a/include/linux/qcom-geni-se.h b/include/linux/qcom-geni-se.h
index dd464943f717..1dc134e9eb36 100644
--- a/include/linux/qcom-geni-se.h
+++ b/include/linux/qcom-geni-se.h
@@ -262,7 +262,7 @@ static inline void geni_se_setup_m_cmd(struct geni_se *se, u32 cmd, u32 params)
 	u32 m_cmd;
 
 	m_cmd = (cmd << M_OPCODE_SHFT) | (params & M_PARAMS_MSK);
-	writel_relaxed(m_cmd, se->base + SE_GENI_M_CMD0);
+	writel(m_cmd, se->base + SE_GENI_M_CMD0);
 }
 
 /**