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Subject: Re: [PATCH 1/3] soc: qcom: geni: More properly switch to DMA mode
To:     Douglas Anderson <dianders@chromium.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Wolfram Sang <wsa@kernel.org>
Cc:     linux-arm-msm@vger.kernel.org, Stephen Boyd <swboyd@chromium.org>,
        linux-i2c@vger.kernel.org, Andy Gross <agross@kernel.org>,
        Girish Mahadevan <girishm@codeaurora.org>,
        Karthikeyan Ramasubramanian <kramasub@codeaurora.org>,
        Mukesh Kumar Savaliya <msavaliy@codeaurora.org>,
        linux-kernel@vger.kernel.org
References: <20201008225235.2035820-1-dianders@chromium.org>
 <20201008155154.1.Ifdb1b69fa3367b81118e16e9e4e63299980ca798@changeid>
From:   Akash Asthana <akashast@codeaurora.org>
Message-ID: <698e9616-cca9-e01d-391e-ad0439c08e04@codeaurora.org>
Date:   Mon, 12 Oct 2020 13:39:36 +0530
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On 10/9/2020 4:22 AM, Douglas Anderson wrote:
> On geni-i2c transfers using DMA, it was seen that if you program the
> command (I2C_READ) before calling geni_se_rx_dma_prep() that it could
> cause interrupts to fire.  If we get unlucky, these interrupts can
> just keep firing (and not be handled) blocking further progress and
> hanging the system.
>
> In commit 02b9aec59243 ("i2c: i2c-qcom-geni: Fix DMA transfer race")
> we avoided that by making sure we didn't program the command until
> after geni_se_rx_dma_prep() was called.  While that avoided the
> problems, it also turns out to be invalid.  At least in the TX case we
> started seeing sporadic corrupted transfers.  This is easily seen by
> adding an msleep() between the DMA prep and the writing of the
> command, which makes the problem worse.  That means we need to revert
> that commit and find another way to fix the bogus IRQs.
>
> Specifically, after reverting commit 02b9aec59243 ("i2c:
> i2c-qcom-geni: Fix DMA transfer race"), I put some traces in.  I found
> that the when the interrupts were firing like crazy:
> - "m_stat" had bits for M_RX_IRQ_EN, M_RX_FIFO_WATERMARK_EN set.
> - "dma" was set.
>
> Further debugging showed that I could make the problem happen more
> reliably by adding an "msleep(1)" any time after geni_se_setup_m_cmd()
> ran up until geni_se_rx_dma_prep() programmed the length.
>
> A rather simple fix is to change geni_se_select_dma_mode() so it's a
> true inverse of geni_se_select_fifo_mode() and disables all the FIFO
> related interrupts.  Now the problematic interrupts can't fire and we
> can program things in the correct order without worrying.
>
> As part of this, let's also change the writel_relaxed() in the prepare
> function to a writel() so that our DMA is guaranteed to be prepared
> now that we can't rely on geni_se_setup_m_cmd()'s writel().
>
> NOTE: the only current user of GENI_SE_DMA in mainline is i2c.
>
> Fixes: 37692de5d523 ("i2c: i2c-qcom-geni: Add bus driver for the Qualcomm GENI I2C controller")
> Fixes: 02b9aec59243 ("i2c: i2c-qcom-geni: Fix DMA transfer race")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Akash Asthana <akashast@codeaurora.org>

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