spi: sh-msiof: Double maximum DMA transfer size using two groups

The maximum DMA transfer size is limited by the maximum values that can
be written to the word count fields (WDLENx) in the Transmit and Control
Data Registers (SITDR2/SIRDR2). As all MSIOF variants support
transferring data of multiple (two or four) groups, the maximum size can
be doubled by using two groups instead of one, thus reducing setup
overhead for very large SPI transfers.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Link: https://patch.msgid.link/bad52[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-sh-msiof.c

···
 }
 
 static int sh_msiof_dma_once(struct sh_msiof_spi_priv *p, const void *tx,
-			     void *rx, unsigned int len)
+			     void *rx, unsigned int len,
+			     unsigned int max_wdlen)
 {
 	u32 ier_bits = 0;
 	struct dma_async_tx_descriptor *desc_tx = NULL, *desc_rx = NULL;
+	unsigned int words1, words2;
 	dma_cookie_t cookie;
 	int ret;
 
···
 		       FIELD_PREP(SIFCTR_RFWM, SIFCTR_RFWM_1));
 
 	/* setup msiof transfer mode registers (32-bit words) */
-	sh_msiof_spi_set_mode_regs(p, tx, rx, 32, len / 4, 0);
+	words1 = min(len / 4, max_wdlen);
+	words2 = len / 4 - words1;
+	sh_msiof_spi_set_mode_regs(p, tx, rx, 32, words1, words2);
 
 	sh_msiof_write(p, SIIER, ier_bits);
 
···
 		 *  DMA supports 32-bit words only, hence pack 8-bit and 16-bit
 		 *  words, with byte resp. word swapping.
 		 */
-		unsigned int l = min(round_down(len, 4), max_wdlen * 4);
+		unsigned int l = min(round_down(len, 4), 2 * max_wdlen * 4);
 
 		if (bits <= 8) {
 			copy32 = copy_bswap32;
···
 		if (tx_buf)
 			copy32(p->tx_dma_page, tx_buf, l / 4);
 
-		ret = sh_msiof_dma_once(p, tx_buf, rx_buf, l);
+		ret = sh_msiof_dma_once(p, tx_buf, rx_buf, l, max_wdlen);
 		if (ret == -EAGAIN) {
 			dev_warn_once(&p->pdev->dev,
 				"DMA not available, falling back to PIO\n");