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Using the UTRLCNR register involves two MMIO accesses in the hot path while
using the doorbell register only involves a single MMIO access. Since MMIO
accesses take time, do not use the UTRLCNR register. The spinlock
contention on the SCSI host lock that is reintroduced by this commit will
be addressed later.

This reverts commit 6f715172.

Link: https://lore.kernel.org/r/20210722033439.26550-12-bvanassche@acm.org


Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Stanley Chu <stanley.chu@mediatek.com>
Cc: Can Guo <cang@codeaurora.org>
Cc: Asutosh Das <asutoshd@codeaurora.org>
Cc: Avri Altman <avri.altman@wdc.com>
Tested-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>