spi: davinci: add comment about dummy tx buffer usage
Add explanation about using the the rx buffer as the dummy tx buffer. Signed-off-by: Frode Isaksen <fisaksen@baylibre.com> Signed-off-by: Mark Brown <broonie@kernel.org>
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@ -660,7 +660,11 @@ static int davinci_spi_bufs(struct spi_device *spi, struct spi_transfer *t)
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goto err_desc;
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goto err_desc;
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if (!t->tx_buf) {
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if (!t->tx_buf) {
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/* use rx buffer as dummy tx buffer */
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/* To avoid errors when doing rx-only transfers with
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* many SG entries (> 20), use the rx buffer as the
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* dummy tx buffer so that dma reloads are done at the
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* same time for rx and tx.
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*/
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t->tx_sg.sgl = t->rx_sg.sgl;
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t->tx_sg.sgl = t->rx_sg.sgl;
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t->tx_sg.nents = t->rx_sg.nents;
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t->tx_sg.nents = t->rx_sg.nents;
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}
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}
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