On Wed, Dec 19, 2018 at 08:41:56AM +0200, Ranran wrote: > On Wed, Dec 19, 2018 at 12:00 AM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Tue, 18 Dec 2018, Ranran wrote: > > > > > Hello, > > > > > > We are afraid that the USB enumeration might take too much time (~3 > > > seconds) , and consider to disable usb and change interface to device > > > from usb to serial. > > > > > > Does anyone knows if it usb enumeration really takes that long in > > > recent kernels ? > > > > USB enumeration time doesn't depend much on the kernel version, but it > > does depend on what USB devices are connected and lots of other things, > > such as how busy the system is. > > > > The best way to find out how long it takes on your system is to measure > > it. > > > > > Is it that usb enumeration at startup can be postponed ? > > > > If you build the USB controller drivers as modules and prevent those > > modules from being loaded at boot time (for example, by moving them > > into a directory different from where they normally belong) then no USB > > enumeration will take place at startup. Some time later you can load > > the modules manually, and enumeration will begin. > > > > But I have to admit, it's hard to see why you would want to postpone > > enumeration if you're worried that it will take too long. I should > > think you'd be more interested in speeding it up than delaying it. > > > > We have strict demands of boot time for up to 2-3 seconds. Boot time to what exactly? And again, have you tested your system? USB devices probe asynchronously, other things will happen while they are being discovered. > We have a usb touch screen, and some of us think that usb enumeration > is problematic (in terms of boot time) and that we better change the > initerface t touch screen from usb to serial. Have you tested this? You need to do that. > Please tell me what you think, is it really better to change the > interface or can we speed up the usb enumeration of this device ? Will > it get into the 2-3 seconds limitation ? Again, you need to test this for your hardware. We have no idea what exactly you are using, nor can we give you any firm answers. You need to do that testing yourself. good luck! greg k-h
