linux-kernel - question about using DMA for davinci SPI controller

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Date:	Mon, 10 May 2010 16:17:03 +0800
From:	Bi Junxiao <junxiao.bi@...driver.com>
To:	davinci-linux-open-source@...ux.davincidsp.com
CC:	linux-kernel@...r.kernel.org
Subject: question about using DMA for davinci SPI controller

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.
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