On Mon, Apr 9, 2012 at 10:14 AM, James Haigh <james.r.haigh@xxxxxxxxx> wrote: > Hello. > > I've been told this is the right place to ask, correct me if I'm wrong. > > I don't think the ability to limit a port to 100mA current draw is > exposed by the drivers. The use-case for this is to avoid a battery > powered device such as a phone from draining the battery of a laptop > while travelling. Data transfer is all that's needed here. It is also > wasteful to charge one battery from another. > > In the interests of saving energy, and prolonging battery-life, please > could a mechanism to restrict to 100mA be implemented. I could then > set up an automatic ACPI response to AC power to allow 500mA and > charge the phone, but battery-to-battery transfer is inefficient and > should be avoided. I am not so sure if this is feasible as there are many non-compliant USB hub and non-compliant USB device out there and Linux needs to support them. http://en.wikipedia.org/wiki/USB_hub > Apparently USB ports can be disabled, and when this occurs no power is > supplied. However, I think the ability to disable a port is also not > exposed by the USB drivers. You can suspend a port. http://www.mjmwired.net/kernel/Documentation/usb/power-management.txt > There are many use-cases for this. In the interest of saving > electricity, it is desirable to unplug unused devices. USB ports can > be plugged only about 1500 times before they ware out, and sometimes > the ports are fairly inaccessible behind the base unit of a computer. > Some badly designed devices will often need resetting by physically > replugging. If a USB device is plugged into a host device that is > remotely monitored, it is not possible to physically reconnect the > device. A less serious and more fun use-case is the automation of USB > 'decorations', which simply use the 5V power supply. Or this crazy USB Hub based AVR programmer. http://www.pjrc.com/hub_isp/ > It seems an oversight that USB devices cannot be programmatically > disconnected and reconnected in GNU/Linux OSes. I would appreciate it > if they could, if possible, and I know for sure that others would too. Maybe you should read this and then see if the current implementations meet your needs. http://www.mjmwired.net/kernel/Documentation/usb/power-management.txt > It has been suggested that I should send a patch in, but I am no > kernel or driver developer, and I'm better off contributing to Free > software projects that are achievable at my skill-level. If you want > me to file these 2 issues/feature-requests somewhere, tell me where > and I will. > -- Xiaofan -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
