spi: omap2-mcspi: Add comments for RX only DMA buffer workaround

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

diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c
index 1d237e9..c47f958 100644 (file)
--- 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;