spi: Add capability to perform some transfer with chipselect off

Some components require a few clock cycles with chipselect off before
or/and after the data transfer done with CS on.

Typically IDT 801034 QUAD PCM CODEC datasheet states "Note *: CCLK
should have one cycle before CS goes low, and two cycles after
CS goes high".

The cycles "before" are implicitely provided by all previous activity
on the SPI bus. But the cycles "after" must be provided in order to
terminate the SPI transfer.

In order to use that kind of component, add a cs_off flag to
spi_transfer struct. When this flag is set, the transfer is performed
with chipselect off. This allows consummer to add a dummy transfer
at the end of the transfer list which is performed with chipselect
OFF, providing the required additional clock cycles.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/434165c46f06d802690208a11e7ea2500e8da4c7.1662558898.git.christophe.leroy@csgroup.euSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1435,7 +1435,8 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 	struct spi_statistics __percpu *statm = ctlr->pcpu_statistics;
 	struct spi_statistics __percpu *stats = msg->spi->pcpu_statistics;
 
-	spi_set_cs(msg->spi, true, false);
+	xfer = list_first_entry(&msg->transfers, struct spi_transfer, transfer_list);
+	spi_set_cs(msg->spi, !xfer->cs_off, false);
 
 	SPI_STATISTICS_INCREMENT_FIELD(statm, messages);
 	SPI_STATISTICS_INCREMENT_FIELD(stats, messages);
@@ -1503,10 +1504,15 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 					 &msg->transfers)) {
 				keep_cs = true;
 			} else {
-				spi_set_cs(msg->spi, false, false);
+				if (!xfer->cs_off)
+					spi_set_cs(msg->spi, false, false);
 				_spi_transfer_cs_change_delay(msg, xfer);
-				spi_set_cs(msg->spi, true, false);
+				if (!list_next_entry(xfer, transfer_list)->cs_off)
+					spi_set_cs(msg->spi, true, false);
 			}
+		} else if (!list_is_last(&xfer->transfer_list, &msg->transfers) &&
+			   xfer->cs_off != list_next_entry(xfer, transfer_list)->cs_off) {
+			spi_set_cs(msg->spi, xfer->cs_off, false);
 		}
 
 		msg->actual_length += xfer->len;

include/linux/spi/spi.h

@@ -847,6 +847,7 @@ struct spi_res {
  *      for this transfer. If 0 the default (from @spi_device) is used.
  * @dummy_data: indicates transfer is dummy bytes transfer.
  * @cs_change: affects chipselect after this transfer completes
+ * @cs_off: performs the transfer with chipselect off.
  * @cs_change_delay: delay between cs deassert and assert when
  *      @cs_change is set and @spi_transfer is not the last in @spi_message
  * @delay: delay to be introduced after this transfer before
@@ -959,6 +960,7 @@ struct spi_transfer {
 	unsigned	cs_change:1;
 	unsigned	tx_nbits:3;
 	unsigned	rx_nbits:3;
+	unsigned	cs_off:1;
 #define	SPI_NBITS_SINGLE	0x01 /* 1bit transfer */
 #define	SPI_NBITS_DUAL		0x02 /* 2bits transfer */
 #define	SPI_NBITS_QUAD		0x04 /* 4bits transfer */