On Thu, Apr 12, 2018 at 05:31:31PM +0200, Jesper Dangaard Brouer wrote:
> > I guess that is because x86 selects it as the default as soon as
> > we have more than 4G memory.
>
> I were also confused why I ended up using SWIOTLB (SoftWare IO-TLB),
> that might explain it. And I'm not hitting the bounce-buffer case.
>
> How do I control which DMA engine I use? (So, I can play a little)
At the lowest level you control it by:
(1) setting the dma_ops pointer in struct device
(2) if that is NULL by choosing what is returned from
get_arch_dma_ops()
>
>
> > That should be solveable fairly easily with the per-device dma ops,
> > though.
>
> I didn't understand this part.
What I mean with that is that we can start out setting dma_ops
to dma_direct_ops for everyone on x86 when we start out (that is assuming
we don't have an iommu), and only switching to swiotlb_dma_ops when
actually required by either a dma_mask that can't address all memory,
or some other special cases like SEV or broken bridges.
> I wanted to ask your opinion, on a hackish idea I have...
> Which is howto detect, if I can reuse the RX-DMA map address, for TX-DMA
> operation on another device (still/only calling sync_single_for_device).
>
> With XDP_REDIRECT we are redirecting between net_device's. Usually
> we keep the RX-DMA mapping as we recycle the page. On the redirect to
> TX-device (via ndo_xdp_xmit) we do a new DMA map+unmap for TX. The
> question is how to avoid this mapping(?). In some cases, with some DMA
> engines (or lack of) I guess the DMA address is actually the same as
> the RX-DMA mapping dma_addr_t already known, right? For those cases,
> would it be possible to just (re)use that address for TX?
You can't in any sensible way without breaking a lot of abstractions.
For dma direct ops that mapping will be the same unless the devices
have different dma_offsets in their struct device, or the architecture
overrides phys_to_dma entirely, in which case all bets are off.
If you have an iommu it depends on which devices are behind the same
iommu.
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