Baochen Qiang <quic_bqiang@xxxxxxxxxxx> wrote: > Currently 32 bit DMA mask is used, telling kernel to get us an DMA > address under 4GB when mapping a buffer. This results in a very high > CPU overhead in the case where IOMMU is disabled and more than 4GB > system memory is installed. The reason is, with more than 4GB memory > installed, kernel is likely to allocate a buffer whose physical > address is above 4GB. While with IOMMU disabled, kernel has to involve > SWIOTLB to map/unmap that buffer, which consumes lots of CPU cycles. > > We did hit an issue caused by the reason mentioned above: in a system > that disables IOMMU and gets 8GB memory installed, a total of 40.5% > CPU usage is observed in throughput test. CPU profiling shows nearly > 60% of CPU cycles are consumed by SWIOTLB. > > By enabling 36 bit DMA mask, we can bypass SWIOTLB for any buffer > whose physical address is below 64GB. There are two types of DMA mask > within struct device, named dma_mask and coherent_dma_mask. Here we > only enable 36 bit for dma_mask, because firmware crashes if > coherent_dma_mask is also enabled, due to some unknown hardware > limitations. This is acceptable because coherent_dma_mask is used for > mapping a consistent DMA buffer, which generally does not happen in > a hot path. > > With this change, the total CPU usage mentioned in above issue drops > to 18.9%. > > Tested-on: WCN6855 hw2.1 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.23 > Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.7.0.1-01744-QCAHKSWPL_SILICONZ-1 > Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.7.0.1-01744-QCAHKSWPL_SILICONZ-1 > > Signed-off-by: Baochen Qiang <quic_bqiang@xxxxxxxxxxx> > Acked-by: Jeff Johnson <quic_jjohnson@xxxxxxxxxxx> > Signed-off-by: Kalle Valo <quic_kvalo@xxxxxxxxxxx> Patch applied to ath-next branch of ath.git, thanks. dbd73acb22d8 wifi: ath11k: enable 36 bit mask for stream DMA -- https://patchwork.kernel.org/project/linux-wireless/patch/20240123015201.28939-1-quic_bqiang@xxxxxxxxxxx/ https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches
