[PATCH] spi: use udelay() for short polling delays

[Peter Collingbourne]

A short polling delay, such as the delay of 5us
(SPINAND_READ_POLL_DELAY_US) provided by the SPI NAND driver,
can become a 1/HZ (order of ms) delay caused by the usleep_range()
call in read_poll_timeout(), significantly reducing SPI NAND access
performance. Fix it by implementing the polling delay with udelay()
(via read_poll_timeout_atomic()) if it is short enough, matching how
the initial delay is handled.

Fixes: c955a0cc8a28 ("spi: spi-mem: add automatic poll status functions")
Signed-off-by: Peter Collingbourne <peter@xxxxxxxxx>
---
 drivers/spi/spi-mem.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
index a09371a075d2..914e35e51cea 100644
--- a/drivers/spi/spi-mem.c
+++ b/drivers/spi/spi-mem.c
@@ -1005,10 +1005,19 @@ int spi_mem_poll_status(struct spi_mem *mem,
 			usleep_range((initial_delay_us >> 2) + 1,
 				     initial_delay_us);
 
-		ret = read_poll_timeout(spi_mem_read_status, read_status_ret,
-					(read_status_ret || ((status) & mask) == match),
-					polling_delay_us, timeout_ms * 1000, false, mem,
-					op, &status);
+		if (polling_delay_us < 10)
+			ret = read_poll_timeout_atomic(
+				spi_mem_read_status, read_status_ret,
+				(read_status_ret || ((status)&mask) == match),
+				polling_delay_us, timeout_ms * 1000, false, mem,
+				op, &status);
+		else
+			ret = read_poll_timeout(
+				spi_mem_read_status, read_status_ret,
+				(read_status_ret || ((status)&mask) == match),
+				polling_delay_us, timeout_ms * 1000, false, mem,
+				op, &status);
+
 		if (read_status_ret)
 			return read_status_ret;
 	}
-- 
2.54.0