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.eu
Signed-off-by: Mark Brown <broonie@kernel.org>

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 97487e1..c582ae4 100644 (file)
--- a/drivers/spi/spi.c
+++ b/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 @@ fallback_pio:
                                         &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;

diff --git a/include/linux/spi/spi.h b/include/linux/spi/spi.h
index e6c73d5..6e6c62c 100644 (file)
--- a/include/linux/spi/spi.h
+++ b/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 */