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[83.28.44.19]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-46c221d9a22sm11308423f8f.19.2026.06.24.17.04.24 (version=TLS1_2 cipher=AES128-SHA bits=128/128); Wed, 24 Jun 2026 17:04:25 -0700 (PDT) Date: Thu, 25 Jun 2026 02:04:21 +0200 From: Michal Pecio To: Xincheng Zhang Cc: cyrozap@gmail.com, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, mathias.nyman@intel.com Subject: Re: [PATCH] usb: xhci-pci: Disable 64-bit DMA for VIA VL805 Message-ID: <20260625020421.2e577a94.michal.pecio@gmail.com> In-Reply-To: <20260624070612.337013-1-zhangxincheng@ultrarisc.com> References: <20260623121847.53749028.michal.pecio@gmail.com> <20260624070612.337013-1-zhangxincheng@ultrarisc.com> Precedence: bulk X-Mailing-List: linux-usb@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 24 Jun 2026 15:06:12 +0800, Xincheng Zhang wrote: > Hi Michal, > > On 2026-06-23 12:18 +0200, Michal Pecio wrote: > > But I don't like this abuse of the quirk. Firstly, it causes > > unnecessary bouncing on systems with >4GB RAM and no IOMMU. > > I found other drivers that use DMA_BIT_MASK(36) or even weirder > > numbers, so it seems that we too could request 64 gigs exactly > > with a bit of driver refactoring. > > Thanks for the review. I totally agree that using `DMA_BIT_MASK(36)` > is a much better approach to avoid bounce buffers and register width > issues. By the way, are you sure that 64GB is the magic number and not 1TB? I booted a common AMD64 box with iommu.forcedac=1 and instantly got IOMMU faults, but the addresses were truncated to 40 bits, not 36. I applied 40 bit DMA mask and my VL805 seems to work. I looked into debugfs and many things are mapped close to 1TB, so I wonder if some chips are better than others or maybe there are particular workloads where VL805 truncates something to 36 bits? I tried a few, including bulk, interrupt, isochronous and USB3 bulk streams. How was this problem found? Do you have >64GB RAM and no IOMMU? Or with IOMMU whose driver allocates mappings above 4GB? Regards, Michal