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

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Message-ID: <Z-EV21KDfMHds73Q@hovoldconsulting.com>
Date: Mon, 24 Mar 2025 09:20:43 +0100
From: Johan Hovold <johan@...nel.org>
To: Baochen Qiang <quic_bqiang@...cinc.com>
Cc: Johan Hovold <johan+linaro@...nel.org>,
	Jeff Johnson <jjohnson@...nel.org>,
	Stephan Gerhold <stephan.gerhold@...aro.org>,
	ath12k@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] wifi: ath12k: extend dma mask to 36 bits

On Mon, Mar 24, 2025 at 04:05:33PM +0800, Baochen Qiang wrote:
> On 3/24/2025 3:40 PM, Johan Hovold wrote:
> > On Mon, Mar 24, 2025 at 11:06:16AM +0800, Baochen Qiang wrote:
> >> 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.

> >>> 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?
> > 
> > The mask should reflect the capability of the device. That may be
> > 64 (or 40) bits, but I've only been able to test using 36 bits.
> 
> are you talking about QCN9274? As far as I know, WCN7850 does not support 40 bits or more.

Or really all devices handled by ath12k since they at least currently
use a common mask.

The T14s has a:

	0004:01:00.0 Network controller: Qualcomm Technologies, Inc WCN785x Wi-Fi 7(802.11be) 320MHz 2x2 [FastConnect 7800] (rev 01)

	ath12k_pci 0004:01:00.0: Hardware name: wcn7850 hw2.0
	ath12k_pci 0004:01:00.0: chip_id 0x2 chip_family 0x4 board_id 0xff soc_id 0x40170200

Sounds like 36 bits is the correct choice for now then.

Johan