RE: [PATCH v2 5/5] net: ep93xx_eth: fix DMA API violations

On Thursday, June 02, 2011 12:00 PM, Mika Westerberg wrote:

Russell King said:

quoted

So, to summarize what its doing:

1. It allocates buffers for rx and tx.
2. It maps them with dma_map_single().
      This transfers ownership of the buffer to the DMA device.
3. In ep93xx_xmit,
3a. It copies the data into the buffer with skb_copy_and_csum_dev()
      This violates the DMA buffer ownership rules - the CPU should
      not be writing to this buffer while it is (in principle) owned
      by the DMA device.
3b. It then calls dma_sync_single_for_cpu() for the buffer.
      This transfers ownership of the buffer to the CPU, which surely
      is the wrong direction.
4. In ep93xx_rx,
4a. It calls dma_sync_single_for_cpu() for the buffer.
      This at least transfers the DMA buffer ownership to the CPU
      before the CPU reads the buffer
4b. It then uses skb_copy_to_linear_data() to copy the data out.
      At no point does it transfer ownership back to the DMA device.
5. When the driver is removed, it dma_unmap_single()'s the buffer.
      This transfers ownership of the buffer to the CPU.
6. It frees the buffer.

While it may work on ep93xx, it's not respecting the DMA API rules,
and with DMA debugging enabled it will probably encounter quite a few
warnings.

This patch fixes these violations.

Signed-off-by: Mika Westerberg <redacted>
Acked-by: Russell King <redacted>

Well... I'm not going to even pretend to actually understand the DMA API
at this point.  But, this patch seems to follow what DMA-API-HOWTO.txt
describes as the proper usage of the DMA API.

Also, with this patch and the others in your series my test systems are
still booting and working properly.

Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>