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    Subject[PATCH RFC v3 11/11] scsi: ufs: core: Perform read back before writing run/stop regs
    Date
    Currently a wmb() is used to ensure that writes to the
    UTP_TASK_REQ_LIST_BASE* regs are completed prior to following writes to
    the run/stop registers.

    wmb() ensure that the write completes, but completion doesn't mean that
    it isn't stored in a buffer somewhere. The recommendation for
    ensuring the bits have taken effect on the device is to perform a read
    back to force it to make it all the way to the device. This is
    documented in device-io.rst and a talk by Will Deacon on this can
    be seen over here:

    https://youtu.be/i6DayghhA8Q?si=MiyxB5cKJXSaoc01&t=1678

    Let's do that to ensure the bits hit the device. Because the wmb()'s
    purpose wasn't to add extra ordering (on top of the ordering guaranteed
    by writel()/readl()), it can safely be removed.

    Fixes: 897efe628d7e ("scsi: ufs: add missing memory barriers")
    Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
    ---
    drivers/ufs/core/ufshcd.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/drivers/ufs/core/ufshcd.c b/drivers/ufs/core/ufshcd.c
    index caebd589e08c..7c1975a1181f 100644
    --- a/drivers/ufs/core/ufshcd.c
    +++ b/drivers/ufs/core/ufshcd.c
    @@ -4726,7 +4726,7 @@ int ufshcd_make_hba_operational(struct ufs_hba *hba)
    * Make sure base address and interrupt setup are updated before
    * enabling the run/stop registers below.
    */
    - wmb();
    + ufshcd_readl(hba, REG_UTP_TASK_REQ_LIST_BASE_H);

    /*
    * UCRDY, UTMRLDY and UTRLRDY bits must be 1
    --
    2.43.0


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