Re: [PATCH] wifi: ath12k: extend dma mask to 36 bits

[Baochen Qiang <quic_bqiang@quicinc.com>]

On 3/22/2025 12:23 AM, Johan Hovold wrote:
> Extend the DMA mask to 36 bits to avoid using bounce buffers on machines
> without an iommu (under OS control) similar to what was done for ath11k
> in commit dbd73acb22d8 ("wifi: ath11k: enable 36 bit mask for stream
> DMA").
> 
> This specifically avoids using bounce buffers on Qualcomm Snapdragon X
> Elite machines like the Lenovo ThinkPad T14s when running at EL1.

why bounce buffer is used at EL1? is it because IOMMU is not working at EL1?
or even because IOMMU is not present on Elite machines?

> 
> Note that the mask could possibly be extended further but unresolved DMA
> issues with 64 GiB X Elite machines currently prevents that from being
> tested.

could you help elaborate how it could be extended?

> 
> Also note that the driver is limited to 32 bits for coherent
> allocations and that there is no need to check for errors when setting
> masks larger than 32 bits.

why is it not necessary to check error?

> 
> Tested-on: WCN7850 hw2.0 WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3
> 
> Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
> ---
>  drivers/net/wireless/ath/ath12k/mhi.c |  2 +-
>  drivers/net/wireless/ath/ath12k/pci.c | 12 ++++--------
>  drivers/net/wireless/ath/ath12k/pci.h |  1 +
>  3 files changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/wireless/ath/ath12k/mhi.c b/drivers/net/wireless/ath/ath12k/mhi.c
> index 2f6d14382ed7..9c929c4ad8a8 100644
> --- a/drivers/net/wireless/ath/ath12k/mhi.c
> +++ b/drivers/net/wireless/ath/ath12k/mhi.c
> @@ -379,7 +379,7 @@ int ath12k_mhi_register(struct ath12k_pci *ab_pci)
>  		mhi_ctrl->irq_flags = IRQF_SHARED | IRQF_NOBALANCING;
>  
>  	mhi_ctrl->iova_start = 0;
> -	mhi_ctrl->iova_stop = 0xffffffff;
> +	mhi_ctrl->iova_stop = ab_pci->dma_mask;
>  	mhi_ctrl->sbl_size = SZ_512K;
>  	mhi_ctrl->seg_len = SZ_512K;
>  	mhi_ctrl->fbc_download = true;
> diff --git a/drivers/net/wireless/ath/ath12k/pci.c b/drivers/net/wireless/ath/ath12k/pci.c
> index 06cff3849ab8..e77c50fc48ca 100644
> --- a/drivers/net/wireless/ath/ath12k/pci.c
> +++ b/drivers/net/wireless/ath/ath12k/pci.c
> @@ -17,7 +17,7 @@
>  #include "debug.h"
>  
>  #define ATH12K_PCI_BAR_NUM		0
> -#define ATH12K_PCI_DMA_MASK		32
> +#define ATH12K_PCI_DMA_MASK		36
>  
>  #define ATH12K_PCI_IRQ_CE0_OFFSET		3
>  
> @@ -874,13 +874,9 @@ static int ath12k_pci_claim(struct ath12k_pci *ab_pci, struct pci_dev *pdev)
>  		goto disable_device;
>  	}
>  
> -	ret = dma_set_mask_and_coherent(&pdev->dev,
> -					DMA_BIT_MASK(ATH12K_PCI_DMA_MASK));
> -	if (ret) {
> -		ath12k_err(ab, "failed to set pci dma mask to %d: %d\n",
> -			   ATH12K_PCI_DMA_MASK, ret);
> -		goto release_region;
> -	}
> +	ab_pci->dma_mask = DMA_BIT_MASK(ATH12K_PCI_DMA_MASK);
> +	dma_set_mask(&pdev->dev, ab_pci->dma_mask);
> +	dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32));
>  
>  	pci_set_master(pdev);
>  
> diff --git a/drivers/net/wireless/ath/ath12k/pci.h b/drivers/net/wireless/ath/ath12k/pci.h
> index 31584a7ad80e..9e89b64b3545 100644
> --- a/drivers/net/wireless/ath/ath12k/pci.h
> +++ b/drivers/net/wireless/ath/ath12k/pci.h
> @@ -116,6 +116,7 @@ struct ath12k_pci {
>  	unsigned long irq_flags;
>  	const struct ath12k_pci_ops *pci_ops;
>  	u32 qmi_instance;
> +	u64 dma_mask;
>  };
>  
>  static inline struct ath12k_pci *ath12k_pci_priv(struct ath12k_base *ab)