OMAP35x and OMAP37x mentions in the McSPI End-of-Transfer Sequences section
that if the McSPI is configured as a Master and only DMA RX is being
performed then the DMA transfer size needs to be reduced by 1 or 2.
This was originally implemented by:
commit 57c5c28dbc83 ("spi: omap2_mcspi rxdma bugfix")
This patch adds comments to clarify what is going on in the code since its
not obvious what problem its addressing.
Signed-off-by: Franklin S Cooper Jr <fcooper@xxxxxx>
---
drivers/spi/spi-omap2-mcspi.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
index 1d237e9..c47f958 100644
--- a/drivers/spi/spi-omap2-mcspi.c
+++ b/drivers/spi/spi-omap2-mcspi.c
@@ -459,6 +459,11 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
count = xfer->len;
dma_count = xfer->len;
+ /*
+ * In the "End-of-Transfer Procedure" section for DMA RX in OMAP35x TRM
+ * it mentions reducing DMA transfer length by one element in master
+ * normal mode.
+ */
if (mcspi->fifo_depth == 0)
dma_count -= es;
@@ -478,6 +483,10 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
dmaengine_slave_config(mcspi_dma->dma_rx, &cfg);
+ /*
+ * Reduce DMA transfer length by one more if McSPI is
+ * configured in turbo mode.
+ */
if ((l & OMAP2_MCSPI_CHCONF_TURBO) && mcspi->fifo_depth == 0)
dma_count -= es;
@@ -507,6 +516,10 @@ omap2_mcspi_rx_dma(struct spi_device *spi, struct spi_transfer *xfer,
if (mcspi->fifo_depth > 0)
return count;
+ /*
+ * Due to the DMA transfer length reduction the missing bytes must
+ * be read manually to receive all of the expected data.
+ */
omap2_mcspi_set_enable(spi, 0);
elements = element_count - 1;
--
2.7.0
--
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