rockchip: spi: Improve SPI read efficiency

The SPI driver is quite slow at reading data. For example, with a 24MHz
clock on gru it achieves a read speed of only 13.9Mbps.

We can correct this by reading the status registers once, then reading as
many bytes as are available before checking the status registers again. It
seems likely that a status register read requires synchronizing with the
SPI FIFO clock domain, which takes a while.

BUG=chrome-os-partner:56556
BRANCH=none
TEST=run on gru and see the speed increase from 13.920 Mbps to 24.712 Mbps

Change-Id: I24aed0c9c6c5445634c4e056922afaee4e9a7b33
Signed-off-by: Martin Roth <martinroth@chromium.org>
Original-Commit-Id: 49c2fc20d7d7d703763e9b0a6f68313a349a84b9
Original-Change-Id: I42745f01f0fe069f6ae26d866004d36bb257e6b2
Original-Signed-off-by: Simon Glass <sjg@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/376945
Original-Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/16582
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/soc/rockchip/common/include/soc/spi.h

@@ -165,6 +165,13 @@ check_member(rockchip_spi, rxdr, 0x800);
 #define SPI_OMOD_SLAVE	0x01
 
 /* --------Bit fields in CTRLR0--------end */
+
+/* TXFLR bits */
+#define TXFLR_LEVEL_MASK	0x3f
+
+/* RXFLR bits */
+#define RXFLR_LEVEL_MASK	0x3f
+
 /* Bit fields in SR, 7 bits */
 #define SR_MASK	0x7f
 #define SR_BUSY	(1 << 0)

src/soc/rockchip/common/spi.c

@@ -235,11 +235,18 @@ static int do_xfer(struct rockchip_spi *regs, const void *dout,
 			xferred = 1;
 		}
 
+		/*
+		 * Try to read as many bytes as are available in one go.
+		 * Reading the status registers probably requires
+		 * sychronizing with the SPI clock which is pretty slow.
+		 */
 		if (*bytes_in && !(sr & SR_RF_EMPT)) {
-			*in_buf = read32(&regs->rxdr) & 0xff;
-			in_buf++;
-			*bytes_in -= 1;
-			xferred = 1;
+			int todo = read32(&regs->rxflr) & RXFLR_LEVEL_MASK;
+
+			*bytes_in -= todo;
+			xferred = todo;
+			while (todo-- > 0)
+				*in_buf++ = read32(&regs->rxdr) & 0xff;
 		}
 
 		min_xfer -= xferred;