spi: sifive: add PM callbacks to support suspend/resume

The patch has been tested on Unmatched using pm_test. The Unmatched board
uses SD over SPI and it was tested by initiating S2RAM cycles for all
devices while reading/writing files at the same time. We found no dropped
connection to the card or corrupted filesystem during test cycles.
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Greentime Hu <greentime.hu@sifive.com>
Link: https://lore.kernel.org/r/20220610074459.3261383-2-andy.chiu@sifive.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-sifive.c

@@ -427,6 +427,44 @@ static int sifive_spi_remove(struct platform_device *pdev)
 	return 0;
 }
 
+static int sifive_spi_suspend(struct device *dev)
+{
+	struct spi_master *master = dev_get_drvdata(dev);
+	struct sifive_spi *spi = spi_master_get_devdata(master);
+	int ret;
+
+	ret = spi_master_suspend(master);
+	if (ret)
+		return ret;
+
+	/* Disable all the interrupts just in case */
+	sifive_spi_write(spi, SIFIVE_SPI_REG_IE, 0);
+
+	clk_disable_unprepare(spi->clk);
+
+	return ret;
+}
+
+static int sifive_spi_resume(struct device *dev)
+{
+	struct spi_master *master = dev_get_drvdata(dev);
+	struct sifive_spi *spi = spi_master_get_devdata(master);
+	int ret;
+
+	ret = clk_prepare_enable(spi->clk);
+	if (ret)
+		return ret;
+	ret = spi_master_resume(master);
+	if (ret)
+		clk_disable_unprepare(spi->clk);
+
+	return ret;
+}
+
+static DEFINE_SIMPLE_DEV_PM_OPS(sifive_spi_pm_ops,
+				sifive_spi_suspend, sifive_spi_resume);
+
+
 static const struct of_device_id sifive_spi_of_match[] = {
 	{ .compatible = "sifive,spi0", },
 	{}
@@ -438,6 +476,7 @@ static struct platform_driver sifive_spi_driver = {
 	.remove = sifive_spi_remove,
 	.driver = {
 		.name = SIFIVE_SPI_DRIVER_NAME,
+		.pm = &sifive_spi_pm_ops,
 		.of_match_table = sifive_spi_of_match,
 	},
 };