Jeff Johnson <quic_jjohnson@xxxxxxxxxxx> writes: > On 1/22/2024 5:52 PM, Baochen Qiang 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> >> --- >> drivers/net/wireless/ath/ath11k/mhi.c | 2 +- >> drivers/net/wireless/ath/ath11k/pci.c | 16 +++++++++++++--- >> drivers/net/wireless/ath/ath11k/pci.h | 1 + >> 3 files changed, 15 insertions(+), 4 deletions(-) > My Qualcomm Innovation Center copyright checker reports: > > drivers/net/wireless/ath/ath11k/mhi.c copyright missing 2024 > drivers/net/wireless/ath/ath11k/pci.c copyright missing 2024 > drivers/net/wireless/ath/ath11k/pci.h copyright missing 2024 > > Kalle can fix this in the pending tree Yup, added those. -- https://patchwork.kernel.org/project/linux-wireless/list/ https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches
