(struct spi_transfer)->rx_buf == (struct spi_transfer)->tx_buf?
I was tracing calls to dma_map_single() and noticed that spi drivers call this function repeatedly on the same buffer address (with no dma_unmap_single() between). Is this really OK?
Looking into it I saw that the spi drivers' tx_buf and rx_buf were equal (same address), and these are "dma mapped" simultaneously in many spi drivers (tx_buf as DMA_TO_DEVICE; rx_buf as DMA_FROM_DEVICE).
If this buffer (tx_buf === rx_buf) is both read from and written to using DMA, then shouldn't they use DMA_BIDIRECTIONAL? Also, DMA_TO_DEVICE mappings should be done before starting the transfer to the device, while DMA_FROM_DEVICE mappings should be done after the device has finished its transfer, if I understand dma_map_single() correctly (because this is just a cache synchronization thing, right?)
This whole thing just seemed strange to me and I'm hoping someone can explain what's going on!
BTW, the declaration of struct spi_transfer
(http://lxr.linux.no/linux+*/include/linux/spi/spi.h#L430) has this comment:
/* it's ok if tx_buf == rx_buf (right?) */
Since spi is always full duplex, I'm guessing that the idea with using only one buffer is that incoming bytes overwrites queued bytes?
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