question about using DMA for davinci SPI controller
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From: Bi Junxiao on 10 May 2010 04:20 Hi all, I am trying to fix a bug about full duplex DMA transfer for SPI controller hang on ti-omapl138 ref borad. I read the DMA setting code of davinci SPI controller. It's in drivers/spi/davinci_spi.c:davinci_spi_bufs_dma(). There is a code snippet which I don't understand. List below: -------------------------------------------------------------------------------- if (t->tx_buf) { t->tx_dma = dma_map_single(&spi->dev, (void *)t->tx_buf, count, DMA_TO_DEVICE); if (dma_mapping_error(&spi->dev, t->tx_dma)) { dev_dbg(sdev, "Unable to DMA map a %d bytes" " TX buffer\n", count); return -ENOMEM; } temp_count = count; } else { /* We need TX clocking for RX transaction */ t->tx_dma = dma_map_single(&spi->dev, (void *)davinci_spi->tmp_buf, count + 1, DMA_TO_DEVICE); if (dma_mapping_error(&spi->dev, t->tx_dma)) { dev_dbg(sdev, "Unable to DMA map a %d bytes" " TX tmp buffer\n", count); return -ENOMEM; } temp_count = count + 1; } ---------------------------------------------------------------------------------- My question is why the transfer count in else logic is count+1 but in if logic it's only count? When I changed it from count��1 to count and use a test program to read a spi flash, I found the DMA rx interrupt can not be triggered. The test program hung for ever. Would anybody have any idea about this? Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |