Re: i.MX31 usb device support and power supply

On Mon, 5 Mar 2007, Valentin Longchamp wrote:

> Now I have a more precise question about the actual implementation of 
> Freescale i.MX31 kernel release (here is the patch: 
> http://bitshrine.org/gpp/linux-2.6.16-mx31-usb-2.patch). It works well, but 
> only with low-current devices (I have tested the ehci support with USB 
> pendrive flash disk). But when I try to plug in some higher current consumers 
> like a wifi pendrive for instance, which needs 300 mA, the configuration is 
> rejected because of this from the choose_configuration function in 
> drivers/usb/core/hub.c :
> 
> > 		/*
> > 		 * The next test may not be as effective as it should be.
> > 		 * Some hubs have errors in their descriptor, claiming
> > 		 * to be self-powered when they are really bus-powered.
> > 		 * We will overestimate the amount of current such hubs
> > 		 * make available for each port.
> > 		 *
> > 		 * This is a fairly benign sort of failure.  It won't
> > 		 * cause us to reject configurations that we should have
> > 		 * accepted.
> > 		 */
> >
> > 		/* Rule out configs that draw too much bus current */
> > 		if (c->desc.bMaxPower * 2 > udev->bus_mA)
> > 			continue;
> 
> >From what I understand (I'm not very familiar with kernel USB code) 
> c->desc.bMaxPower is what the hub makes available for each port and 
> udev->bus_mA is what the device requests.

It's the reverse: (c->desc.bMaxPower * 2) is what the device requests and 
udev->bus_mA is what the hub makes available.

>  This has failed for me with a 
> device requesting 300 mA.
> 
> Now I was not able to find in the code where this limit is fixed for my ehci 
> hardware. Because I haven't found a current limitation in the USB block or 
> transceiver (Phillips ISP1504) documentation (quick look).

Search for "power_budget" in the source code for your bus glue.

Alan Stern


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