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Message-ID: <73e9ddd2-7c0c-4a96-a120-7480ebffc4f0@quicinc.com>
Date: Tue, 23 Jan 2024 18:35:47 -0800
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Subject: Re: [PATCH] wifi: ath11k: enable 36 bit mask for stream DMA
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To: Baochen Qiang <quic_bqiang@quicinc.com>, <ath11k@lists.infradead.org>
CC: <linux-wireless@vger.kernel.org>
References: <20240123015201.28939-1-quic_bqiang@quicinc.com>
From: Jeff Johnson <quic_jjohnson@quicinc.com>
In-Reply-To: <20240123015201.28939-1-quic_bqiang@quicinc.com>
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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@quicinc.com>
> ---
>  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, actual code LGTM
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>