spi: s3c64xx: Fix large transfers with DMA

The COUNT_VALUE in the PACKET_CNT register is 16-bit so the maximum
value is 65535.  Asking the driver to transfer a larger size currently
leads to the DMA transfer timing out.  Implement ->max_transfer_size()
and have the core split the transfer as needed.

Fixes: 230d42d ("spi: Add s3c64xx SPI Controller driver")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20220927112117.77599-5-vincent.whitchurch@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: vwax

drivers/spi/spi-s3c64xx.c

@@ -84,6 +84,7 @@
 #define S3C64XX_SPI_ST_TX_FIFORDY		(1<<0)
 
 #define S3C64XX_SPI_PACKET_CNT_EN		(1<<16)
+#define S3C64XX_SPI_PACKET_CNT_MASK		GENMASK(15, 0)
 
 #define S3C64XX_SPI_PND_TX_UNDERRUN_CLR		(1<<4)
 #define S3C64XX_SPI_PND_TX_OVERRUN_CLR		(1<<3)
@@ -711,6 +712,13 @@ static int s3c64xx_spi_prepare_message(struct spi_master *master,
 	return 0;
 }
 
+static size_t s3c64xx_spi_max_transfer_size(struct spi_device *spi)
+{
+	struct spi_controller *ctlr = spi->controller;
+
+	return ctlr->can_dma ? S3C64XX_SPI_PACKET_CNT_MASK : SIZE_MAX;
+}
+
 static int s3c64xx_spi_transfer_one(struct spi_master *master,
 				    struct spi_device *spi,
 				    struct spi_transfer *xfer)
@@ -1152,6 +1160,7 @@ static int s3c64xx_spi_probe(struct platform_device *pdev)
 	master->unprepare_transfer_hardware = s3c64xx_spi_unprepare_transfer;
 	master->prepare_message = s3c64xx_spi_prepare_message;
 	master->transfer_one = s3c64xx_spi_transfer_one;
+	master->max_transfer_size = s3c64xx_spi_max_transfer_size;
 	master->num_chipselect = sci->num_cs;
 	master->use_gpio_descriptors = true;
 	master->dma_alignment = 8;