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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 57c5c28dbc ("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>
This commit is contained in:
Franklin S Cooper Jr 2016-06-27 09:54:08 -05:00 committed by Mark Brown
parent 1a695a905c
commit 4bd00413cd
1 changed files with 13 additions and 0 deletions
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13
drivers/spi/spi-omap2-mcspi.c
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@ -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;