Comment 4 for bug 1859744

In linux-next but not hit mainline yet.

$ git show bfda044533b2
commit bfda044533b213985bc62bd7ca96f2b984d21b80
Author: Xinwei Kong <email address hidden>
Date: Fri Jan 3 10:52:10 2020 +0800

    spi: dw: use "smp_mb()" to avoid sending spi data error

    Because of out-of-order execution about some CPU architecture,
    In this debug stage we find Completing spi interrupt enable ->
    prodrucing TXEI interrupt -> running "interrupt_transfer" function
    will prior to set "dw->rx and dws->rx_end" data, so this patch add
    memory barrier to enable dw->rx and dw->rx_end to be visible and
    solve to send SPI data error.
    eg:
    it will fix to this following low possibility error in testing environment
    which using SPI control to connect TPM Modules

    kernel: tpm tpm0: Operation Timed out
    kernel: tpm tpm0: tpm_relinquish_locality: : error -1

    Signed-off-by: fengsheng <email address hidden>
    Signed-off-by: Xinwei Kong <email address hidden>
    Link: https://lore<email address hidden>
    Signed-off-by: Mark Brown <email address hidden>

diff --git a/drivers/spi/spi-dw.c b/drivers/spi/spi-dw.c
index c547ae38ed69..310492e21b33 100644
--- a/drivers/spi/spi-dw.c
+++ b/drivers/spi/spi-dw.c
@@ -297,6 +297,9 @@ static int dw_spi_transfer_one(struct spi_controller *master,
        dws->len = transfer->len;
        spin_unlock_irqrestore(&dws->buf_lock, flags);

+ /* Ensure dw->rx and dw->rx_end are visible */
+ smp_mb();
+
        spi_enable_chip(dws, 0);

        /* Handle per transfer options for bpw and speed */