spi: atmel-quadspi: Add support for configuring CS timing

The at91 QSPI IP uses a default value of half of the period of the QSPI
clock period for the cs-setup time, which is not always enough, an example
being the sst26vf064b SPI NOR flash which requires a minimum cs-setup time
of 5 ns. It was observed that none of the at91 SoCs can fulfill the
minimum CS setup time for the aforementioned flash, as they operate at
high frequencies and half a period does not suffice for the required CS
setup time. Add support for configuring the CS timing in the controller.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20221117105249.115649-5-tudor.ambarus@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: ambarus

drivers/spi/atmel-quadspi.c

@@ -510,6 +510,39 @@ static int atmel_qspi_setup(struct spi_device *spi)
 	return 0;
 }
 
+static int atmel_qspi_set_cs_timing(struct spi_device *spi)
+{
+	struct spi_controller *ctrl = spi->master;
+	struct atmel_qspi *aq = spi_controller_get_devdata(ctrl);
+	unsigned long clk_rate;
+	u32 cs_setup;
+	int delay;
+	int ret;
+
+	delay = spi_delay_to_ns(&spi->cs_setup, NULL);
+	if (delay <= 0)
+		return delay;
+
+	clk_rate = clk_get_rate(aq->pclk);
+	if (!clk_rate)
+		return -EINVAL;
+
+	cs_setup = DIV_ROUND_UP((delay * DIV_ROUND_UP(clk_rate, 1000000)),
+				1000);
+
+	ret = pm_runtime_resume_and_get(ctrl->dev.parent);
+	if (ret < 0)
+		return ret;
+
+	aq->scr |= QSPI_SCR_DLYBS(cs_setup);
+	atmel_qspi_write(aq->scr, aq, QSPI_SCR);
+
+	pm_runtime_mark_last_busy(ctrl->dev.parent);
+	pm_runtime_put_autosuspend(ctrl->dev.parent);
+
+	return 0;
+}
+
 static void atmel_qspi_init(struct atmel_qspi *aq)
 {
 	/* Reset the QSPI controller */
@@ -555,6 +588,7 @@ static int atmel_qspi_probe(struct platform_device *pdev)
 
 	ctrl->mode_bits = SPI_RX_DUAL | SPI_RX_QUAD | SPI_TX_DUAL | SPI_TX_QUAD;
 	ctrl->setup = atmel_qspi_setup;
+	ctrl->set_cs_timing = atmel_qspi_set_cs_timing;
 	ctrl->bus_num = -1;
 	ctrl->mem_ops = &atmel_qspi_mem_ops;
 	ctrl->num_chipselect = 1;