linux-kernel - Re: [PATCH v2] i2c: i2c-qcom-geni: Fix DMA transfer race

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Date:   Wed, 22 Jul 2020 17:50:11 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>,
        Wolfram Sang <wsa@...-dreams.de>
Cc:     msavaliy@...eaurora.org,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Akash Asthana <akashast@...eaurora.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Douglas Anderson <dianders@...omium.org>,
        Alok Chauhan <alokc@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Girish Mahadevan <girishm@...eaurora.org>,
        Karthikeyan Ramasubramanian <kramasub@...eaurora.org>,
        Sagar Dharia <sdharia@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] i2c: i2c-qcom-geni: Fix DMA transfer race

Quoting Douglas Anderson (2020-07-22 15:00:21)
> 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 starting 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.
> 
> Although problems were seen on the RX path, code inspection suggests
> that the TX should be changed too.  Change it as well.
> 
> Fixes: 37692de5d523 ("i2c: i2c-qcom-geni: Add bus driver for the Qualcomm GENI I2C controller")
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
> Reviewed-by: Akash Asthana <akashast@...eaurora.org>
> ---

Reviewed-by: Stephen Boyd <swboyd@...omium.org>