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

[Douglas Anderson]

When I have KASAN enabled on my kernel and I start stressing the
touchscreen my system tends to hang.  The touchscreen is one of the
only things that does a lot of big i2c transfers and ends up hitting
the DMA paths in the geni i2c driver.  It appears that KASAN adds
enough delay in my system to tickle a race condition in the DMA setup
code.

When the system hangs, I found that it was running the geni_i2c_irq()
over and over again.  It had these:

m_stat   = 0x04000080
rx_st    = 0x30000011
dm_tx_st = 0x00000000
dm_rx_st = 0x00000000
dma      = 0x00000001

Notably we're in DMA mode but are getting M_RX_IRQ_EN and
M_RX_FIFO_WATERMARK_EN over and over again.

Putting some traces in geni_i2c_rx_one_msg() showed that when we
failed we were getting to the start of geni_i2c_rx_one_msg() but were
never executing geni_se_rx_dma_prep().

I believe that the problem here is that we are writing the transfer
length and setting up the geni command before we run
geni_se_rx_dma_prep().  If a transfer makes it far enough before we do
that then we get into the state I have observed.  Let's change the
order, which seems to work fine.

Fixes: 37692de5d523 ("i2c: i2c-qcom-geni: Add bus driver for the Qualcomm GENI I2C controller")
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 drivers/i2c/busses/i2c-qcom-geni.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/i2c/busses/i2c-qcom-geni.c b/drivers/i2c/busses/i2c-qcom-geni.c
index 18d1e4fd4cf3..21e27f10510a 100644
--- a/drivers/i2c/busses/i2c-qcom-geni.c
+++ b/drivers/i2c/busses/i2c-qcom-geni.c
@@ -366,15 +366,15 @@ static int geni_i2c_rx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
 	else
 		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);
 		i2c_put_dma_safe_msg_buf(dma_buf, msg, false);
 		dma_buf = NULL;
 	}
 
+	writel_relaxed(len, se->base + SE_I2C_RX_TRANS_LEN);
+	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);
-- 
2.28.0.rc0.142.g3c755180ce-goog