spi: ath79: add mem_ops for fast-read

Reading from memory is 3x faster than bit-bang read operation. Also,
for tl-wr2543nd, the bit-bang read was sporadically returning random
data, possibly a HW defect, while fast-read works as expected.
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Link: https://lore.kernel.org/r/20220129040453.8476-1-luizluca@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-ath79.c

@@ -15,6 +15,7 @@
 #include <linux/platform_device.h>
 #include <linux/io.h>
 #include <linux/spi/spi.h>
+#include <linux/spi/spi-mem.h>
 #include <linux/spi/spi_bitbang.h>
 #include <linux/bitops.h>
 #include <linux/clk.h>
@@ -133,6 +134,38 @@ static u32 ath79_spi_txrx_mode0(struct spi_device *spi, unsigned int nsecs,
 	return ath79_spi_rr(sp, AR71XX_SPI_REG_RDS);
 }
 
+static int ath79_exec_mem_op(struct spi_mem *mem,
+			     const struct spi_mem_op *op)
+{
+	struct ath79_spi *sp = ath79_spidev_to_sp(mem->spi);
+
+	/* Ensures that reading is performed on device connected to hardware cs0 */
+	if (mem->spi->chip_select || mem->spi->cs_gpiod)
+		return -ENOTSUPP;
+
+	/* Only use for fast-read op. */
+	if (op->cmd.opcode != 0x0b || op->data.dir != SPI_MEM_DATA_IN ||
+	    op->addr.nbytes != 3 || op->dummy.nbytes != 1)
+		return -ENOTSUPP;
+
+	/* disable GPIO mode */
+	ath79_spi_wr(sp, AR71XX_SPI_REG_FS, 0);
+
+	memcpy_fromio(op->data.buf.in, sp->base + op->addr.val, op->data.nbytes);
+
+	/* enable GPIO mode */
+	ath79_spi_wr(sp, AR71XX_SPI_REG_FS, AR71XX_SPI_FS_GPIO);
+
+	/* restore IOC register */
+	ath79_spi_wr(sp, AR71XX_SPI_REG_IOC, sp->ioc_base);
+
+	return 0;
+}
+
+static const struct spi_controller_mem_ops ath79_mem_ops = {
+	.exec_op = ath79_exec_mem_op,
+};
+
 static int ath79_spi_probe(struct platform_device *pdev)
 {
 	struct spi_master *master;
@@ -154,6 +187,7 @@ static int ath79_spi_probe(struct platform_device *pdev)
 	master->bits_per_word_mask = SPI_BPW_RANGE_MASK(1, 32);
 	master->flags = SPI_MASTER_GPIO_SS;
 	master->num_chipselect = 3;
+	master->mem_ops = &ath79_mem_ops;
 
 	sp->bitbang.master = master;
 	sp->bitbang.chipselect = ath79_spi_chipselect;