On Mon, Feb 10, 2020 at 3:12 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > 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 > Alan, Thank you for the swift response. On the first point, if we modify the kernel to re-enumerate the same bMaxPower of 500mA would this still be a problem? ie is there a period when the kernel takes over the USB controller that the Host side would go back to 100mA after U-Boot negotiates the 500mA? The last point you make is a very good one and something I hadn't thought of. If we can overcome the first hurdle I think we would be happy for it to be a requirement for the device to be plugged directly into a PC.
