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Fri, 15 Aug 2025 19:04:10 -0700 (PDT) Received: from rowland.harvard.edu ([2601:19b:681:fd10::f777]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4b11dc5d069sm17884101cf.21.2025.08.15.19.04.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 15 Aug 2025 19:04:10 -0700 (PDT) Date: Fri, 15 Aug 2025 22:04:07 -0400 From: Alan Stern To: ValdikSS Cc: linux-usb@vger.kernel.org Subject: Re: USB 1.1 Full Speed OHCI slow/high latency Message-ID: <4e83d098-19e8-45aa-94e7-9ef005a0c8ab@rowland.harvard.edu> References: <3fe845b9-1328-4b40-8b02-61a879bea6df@valdikss.org.ru> <6017298f-fc03-41c9-b0e3-a74180f4c9a1@rowland.harvard.edu> <2bf5c54e-7dac-4673-a696-e09eb8d459d5@valdikss.org.ru> <7bfaeea9-6eb1-4e49-af8a-77940fdb70fc@rowland.harvard.edu> <9013fce0-6764-49b1-9e54-68e915e12d7c@valdikss.org.ru> 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: <9013fce0-6764-49b1-9e54-68e915e12d7c@valdikss.org.ru> On Fri, Aug 15, 2025 at 09:13:09PM +0300, ValdikSS wrote: > I *buy* these legacy devices to improve them in Linux :) > > I'm making an open-source print server hardware for older printers, that's > why for me this issue is worth researching, at least to document such > hardware heisenbugs. > There've been different USB behavior and incompatibilities with scanners > (fixed it in SANE) and printers (fixed by CUPS quirks file), but this is the > first time I'm going down to USB HC level. > > This printer (Canon LBP1120) does not have drivers for 64-bit Windows or > recent macOS, that's why for many it's a paperweight device you can't use or > sell, unless fixed by other means (usually a VM with 32-bit Windows, and now > my print server as well). > > You gave me directions where I should look for, because I didn't know how to > start debugging without the logic analyzer. > Usbmon captures only USB communication packets and some of USB HUB > controlling packets (such as TT related), but does not include > device-controlling MMIO registers, which was the case here. > > I'll add debug prints to everything related to MMIO and take a look what > could be improved. Thanks! Good luck! > If you have other ideas I could try right away, please share them! You might compare the usbmon traces for your patched OHCI against a UHCI system. They should run at approximately the same speed, so it's not clear why the printer would work with one but not the other. Alan Stern