spi: imx: Implement support for CS_WORD

This only works when the native chipselect is in use. On a board with a
Ti ADS7950 8 channel ADC. This patch reduces the time to read out all
channels once from 280 us to 20 us.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210727124226.5571-1-u.kleine-koenig@pengutronix.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-imx.c

@@ -1176,6 +1176,7 @@ static int spi_imx_setupxfer(struct spi_device *spi,
 	 * dynamic_burst in that case.
 	 */
 	if (spi_imx->devtype_data->dynamic_burst && !spi_imx->slave_mode &&
+	    !(spi->mode & SPI_CS_WORD) &&
 	    (spi_imx->bits_per_word == 8 ||
 	    spi_imx->bits_per_word == 16 ||
 	    spi_imx->bits_per_word == 32)) {
@@ -1611,6 +1612,15 @@ static int spi_imx_probe(struct platform_device *pdev)
 	    is_imx53_ecspi(spi_imx))
 		spi_imx->bitbang.master->mode_bits |= SPI_LOOP | SPI_READY;
 
+	if (is_imx51_ecspi(spi_imx) &&
+	    device_property_read_u32(&pdev->dev, "cs-gpios", NULL))
+		/*
+		 * When using HW-CS implementing SPI_CS_WORD can be done by just
+		 * setting the burst length to the word size. This is
+		 * considerably faster than manually controlling the CS.
+		 */
+		spi_imx->bitbang.master->mode_bits |= SPI_CS_WORD;
+
 	spi_imx->spi_drctl = spi_drctl;
 
 	init_completion(&spi_imx->xfer_done);