From 4bd00413cde851d84b297c1b0dae15109025e84b Mon Sep 17 00:00:00 2001 From: Franklin S Cooper Jr Date: Mon, 27 Jun 2016 09:54:08 -0500 Subject: 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 Signed-off-by: Mark Brown --- drivers/spi/spi-omap2-mcspi.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'drivers/spi') diff --git a/drivers/spi/spi-omap2-mcspi.c b/drivers/spi/spi-omap2-mcspi.c index 1d237e93a289..c47f95879833 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; -- cgit v1.2.1