drm/mipi-dbi: align max_chunk to 2 in spi_transfer

In __spi_validate, there's a validation that no partial transfers
are accepted (xfer->len % w_size must be zero). When
max_chunk is not a multiple of bpw (e.g. max_chunk = 65535,
bpw = 16), the transfer will be rejected.

This patch aligns max_chunk to 2 bytes (the maximum value of bpw is 16),
so that no partial transfer will occur.

Fixes: d23d4d4d ("drm/tinydrm: Move tinydrm_spi_transfer()")
Signed-off-by: Yunhao Tian <t123yh.xyz@gmail.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220510030219.2486687-1-t123yh.xyz@gmail.com

drivers/gpu/drm/drm_mipi_dbi.c

@@ -1199,6 +1199,13 @@ int mipi_dbi_spi_transfer(struct spi_device *spi, u32 speed_hz,
 	size_t chunk;
 	int ret;
 
+	/* In __spi_validate, there's a validation that no partial transfers
+	 * are accepted (xfer->len % w_size must be zero).
+	 * Here we align max_chunk to multiple of 2 (16bits),
+	 * to prevent transfers from being rejected.
+	 */
+	max_chunk = ALIGN_DOWN(max_chunk, 2);
+
 	spi_message_init_with_transfers(&m, &tr, 1);
 
 	while (len) {