spi: amd: Limit max transfer and message size

Enabling the SPI CS35L41 audio codec driver for Steam Deck [1]
revealed a problem with the current AMD SPI controller driver
implementation, consisting of an unrecoverable system hang.

The issue can be prevented if we ensure the max transfer size
and the max message size do not exceed the FIFO buffer size.

According to the implementation of the downstream driver, the
AMD SPI controller is not able to handle more than 70 bytes per
transfer, which corresponds to the size of the FIFO buffer.

Hence, let's fix this by setting the SPI limits mentioned above.

[1] https://lore.kernel.org/r/20220621213819.262537-1-cristian.ciocaltea@collabora.comReported-by: Anastasios Vacharakis <vacharakis@o2mail.de>
Fixes: bbb336f3 ("spi: spi-amd: Add AMD SPI controller driver support")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20220706100626.1234731-2-cristian.ciocaltea@collabora.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-amd.c

@@ -33,6 +33,7 @@
 #define AMD_SPI_RX_COUNT_REG	0x4B
 #define AMD_SPI_STATUS_REG	0x4C
 
+#define AMD_SPI_FIFO_SIZE	70
 #define AMD_SPI_MEM_SIZE	200
 
 /* M_CMD OP codes for SPI */
@@ -270,6 +271,11 @@ static int amd_spi_master_transfer(struct spi_master *master,
 	return 0;
 }
 
+static size_t amd_spi_max_transfer_size(struct spi_device *spi)
+{
+	return AMD_SPI_FIFO_SIZE;
+}
+
 static int amd_spi_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
@@ -302,6 +308,8 @@ static int amd_spi_probe(struct platform_device *pdev)
 	master->flags = SPI_MASTER_HALF_DUPLEX;
 	master->setup = amd_spi_master_setup;
 	master->transfer_one_message = amd_spi_master_transfer;
+	master->max_transfer_size = amd_spi_max_transfer_size;
+	master->max_message_size = amd_spi_max_transfer_size;
 
 	/* Register the controller with SPI framework */
 	err = devm_spi_register_master(dev, master);