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 02b9aec5 ("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 02b9aec5 ("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: 37692de5 ("i2c: i2c-qcom-geni: Add bus driver for the Qualcomm GENI I2C controller")
Fixes: 02b9aec5 ("i2c: i2c-qcom-geni: Fix DMA transfer race")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Akash Asthana <akashast@codeaurora.org>
Tested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201013142448.v2.1.Ifdb1b69fa3367b81118e16e9e4e63299980ca798@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>