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Sat, 04 Apr 2026 08:48:43 -0700 (PDT) Received: from rowland.harvard.edu ([2601:19b:d01:d210::5a82]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8a596915986sm83673276d6.24.2026.04.04.08.48.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 04 Apr 2026 08:48:43 -0700 (PDT) Date: Sat, 4 Apr 2026 11:48:40 -0400 From: Alan Stern To: Ajay Garg Cc: linux-usb@vger.kernel.org Subject: Re: Some queries on eHCI/xHCI/Thunderbolt and dwc2/dwc3 Message-ID: References: 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-Disposition: inline In-Reply-To: On Sat, Apr 04, 2026 at 12:26:35PM +0530, Ajay Garg wrote: > Hi everyone. > > I have been gaining an architectural understanding on how host/gadget > usb-drivers inter-operate with USB-2 / USB-3 / USB-4 (host-side) and > synopsys-DWC controllers (gadget-side). > > Following have been my understandings : > > a) > For host-side, eHCI does not require / use dwc2, but xHCI mandatorily > needs dwc3. > > Am I correct? No. EHCI (not "eHCI") is an architecture for a USB-2 host controller. Several companies make controllers that are compatible with the EHCI specification, but Synopsys's DesignWare dwc2 is not one of them. Similarly, xHCI is an architecture for a USB-3 host controller. The DesignWare dwc3 is such a controller, but there are others (such as Intel's). > If yes, how is hardware-specific actions taken care by eHCI alone > (since eHCI is hardware-agnostic)? Part of the EHCI specification says how the host controller should interact with the PCI bus. Most EHCI implementations use PCI, but not all. For those that don't, the ehci-hcd driver has platform-specific submodules. > Is it that USB-2 PHY-registers are already a part of eHCI > MMIO-registers themselves (thus not requiring any hardware-specific > actions on the PHY side)? More or less. The PHY functionality is part of EHCI, but its registers don't correspond very well to those of standalone PHYs. > b) > For host-side, does thunderbolt driver require any of dwc3 / upcoming-dwc4? I don't know anything about thunderbolt, USB-4, or dwc4. > c) > For gadget-side, dwc2 and dwc3 are enough for USB-2 / USB-3. > Am I correct? Yes, as far as I know. But of course there are alternatives. Alan Stern > d) > For gadget-side, would USB-4 require an upcoming-dwc4? Or will dwc3 be > modified? Or is there another plan in the pipeline? > > > Will be grateful for light on the above from kernel/synopsys experts :) > Many thanks in advance ! > > Thanks and Regards, > Ajay