On Mon, 10 Feb 2020, Martin Townsend wrote: > Hi, > > We are using the USB mass storage gadget driver in Linux and > everything is working fairly well but sometimes we are finding that we > are exceeding the 100mA limit (which I think is the default the host > will provide) before Linux has had a change to enumerate the USB > device at which point we ask for 500mA. We have tried to reduce the > power by ensuring all clocks are turned off, devices not used disabled > etc but can't seem to satisfy the 100mA limit. We were thinking that > maybe we could enumerate the USB device during U-Boot at which point > we know we are under the 100mA limit. Does anyone know a reason why > this would not work? It won't work because U-Boot isn't the kernel. When the kernel takes over the USB controller, it will force the host to re-enumerate the gadget -- and you will be right back where you started. The only way to make this work would be to prevent U-Boot from ever actually booting the Linux kernel. This probably isn't the way you want your device to behave, though. > would at some point from U-Boot enumerating a new > bMaxPower would the connection be reset and drop back to 100mA during > the kernel startup? Are there any considerations that we would need to > take care of in the kernel for this to work? If you can't satisfy the 100-mA current limit then you should make your device be self-powered instead of relying on power delivered over the USB bus. Be aware also that bus-powered USB-2 hubs can't deliver 500 mA to their downstream ports. They are always limited to 100 mA or less. If your gadget was plugged into one of them, it wouldn't be able to work at all. Alan Stern
