On 09/05/2024 2:43 pm, Steven Price wrote:
On 07/05/2024 12:20, Alexander Lobakin wrote:
Quite often, devices do not need dma_sync operations on x86_64 at least.
Indeed, when dev_is_dma_coherent(dev) is true and
dev_use_swiotlb(dev) is false, iommu_dma_sync_single_for_cpu()
and friends do nothing.
However, indirectly calling them when CONFIG_RETPOLINE=y consumes about
10% of cycles on a cpu receiving packets from softirq at ~100Gbit rate.
Even if/when CONFIG_RETPOLINE is not set, there is a cost of about 3%.
Add dev->need_dma_sync boolean and turn it off during the device
initialization (dma_set_mask()) depending on the setup:
dev_is_dma_coherent() for the direct DMA, !(sync_single_for_device ||
sync_single_for_cpu) or the new dma_map_ops flag, %DMA_F_CAN_SKIP_SYNC,
advertised for non-NULL DMA ops.
Then later, if/when swiotlb is used for the first time, the flag
is reset back to on, from swiotlb_tbl_map_single().
On iavf, the UDP trafficgen with XDP_DROP in skb mode test shows
+3-5% increase for direct DMA.
Suggested-by: Christoph Hellwig <hch@xxxxxx> # direct DMA shortcut
Co-developed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
I've bisected a boot failure (on a Firefly RK3288) to this commit.
AFAICT the problem is that I have (at least) two drivers which don't
call dma_set_mask() and therefore never initialise the new dma_need_sync
variable.
The specific drivers are "rockchip-drm" and "rk_gmac-dwmac". Is it a
requirement that all drivers engaging in DMA should call dma_set_mask()
- and therefore this has uncovered a bug in those drivers. Or is the
assumption that all drivers call dma_set_mask() faulty?
Historically it's long been documented (at least in DMA-API-HOWTO) that
a 32-bit DMA mask is assumed by default, so as much as we would prefer
to shift expectations, there are still going to be a great many drivers
relying on that :(
Perhaps its time for dma-debug to start warning about implicit mask
usage, maybe that might help push the agenda a bit?
Thanks,
Robin.
