UPSTREAM: scsi: ufs: core: Remove redundant wmb() in ufshcd_send_command()

The wmb() inside ufshcd_send_command() is added to make sure that the
doorbell is committed immediately. This leads to couple of expectations:

 1. The doorbell write should complete before the function return.

 2. The doorbell write should not cross the function boundary.

2nd expectation is fullfilled by the Linux memory model as there is a
guarantee that the critical section won't cross the unlock (release)
operation.

1st expectation is not really needed here as there is no following read/
write that depends on the doorbell to be complete implicitly. Even if the
doorbell write is in a CPUs Write Buffer (WB), wmb() won't flush it. And
there is no real need of a WB flush here as well.

So let's get rid of the wmb() that seems redundant.

Link: https://lore.kernel.org/r/20220504084212.11605-5-manivannan.sadhasivam@linaro.org
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Bug: 234653003
(cherry picked from commit 23803bacaa)
Change-Id: I237ad3fec8301aa9d17c508b1c50bff019eeae6d
Signed-off-by: Bart Van Assche <bvanassche@google.com>

drivers/scsi/ufs/ufshcd.c

@@ -2189,9 +2189,6 @@ void ufshcd_send_command(struct ufs_hba *hba, unsigned int task_tag)
 	__set_bit(task_tag, &hba->outstanding_reqs);
 	ufshcd_writel(hba, 1 << task_tag, REG_UTP_TRANSFER_REQ_DOOR_BELL);
 	spin_unlock_irqrestore(&hba->outstanding_lock, flags);
-
-	/* Make sure that doorbell is committed immediately */
-	wmb();
 }
 
 /**