On Wed, 5 Dec 2012, Sebastian Andrzej Siewior wrote:
> On Mon, Dec 03, 2012 at 04:28:38PM -0500, Alan Stern wrote:
> > On Mon, 3 Dec 2012, Sarah Sharp wrote:
> > > This code in hcd.c:usb_add_hcd needs to change:
> > >
> > > /* starting here, usbcore will pay attention to this root hub */
> > > rhdev->bus_mA = min(500u, hcd->power_budget);
> >
> > In fact, this line is wrong. The 500u should be multiplied by the
> > number of ports -- and of course, it should be larger for SuperSpeed
> > root hubs.
>
> But I don't know the number of ports here.
Ah, right. That's probably a large part of the reason it wasn't done
correctly. Or maybe I just wasn't thinking clearly when I wrote it...
> I could set this here to 500/900
> and multiple later in hub_configure() by the number of ports.
Better yet, move that entire bus_mA calculation from usb_add_hcd to
hub_configure. It should be something like this:
if (hdev == hdev->bus->root_hub) {
+ if (hcd->power_budget > 0)
+ hdev->bus_mA = hcd->power_budget;
+ else
+ hdev->bus_mA = full_load * hdev->maxchild;
+ if (hdev->bus_mA >= full_load)
- if (hdev->bus_mA == 0 || hdev->bus_mA >= full_load)
hub->mA_per_port = full_load;
else {
hub->mA_per_port = hdev->bus_mA;
hub->limited_power = 1;
}
Leaving rhdev->bus_mA set to 0 for a while shouldn't hurt anything,
because our config descriptors for root hubs all have bMaxPower equal
to 0.
In case this isn't clear (which seems quite likely), hdev->bus_mA
represents the total amount of current available for use by the hub
itself plus its downstream devices. So on a regular PC, where each
root-hub port can supply a full load, we say that the root hub uses no
power and hdev->bus_mA is a full load times the number of ports.
This all makes more sense if you think about external hubs.
> Anyway, do you think this is correct? Anything > 500 for non-SS will replaced
> with 500 in hub_configure:
>
> | if (hdev == hdev->bus->root_hub) {
> | if (hdev->bus_mA == 0 || hdev->bus_mA >= full_load)
> | hub->mA_per_port = full_load;
> | else {
> | hub->mA_per_port = hdev->bus_mA;
> | hub->limited_power = 1;
> | }
True, hub->mA_per_port isn't affected by the multiplication. But look
at the calculation in hub_power_remaining -- it won't work right if
hdev->bus_mA isn't set correctly.
> going through the tree I saw most people set it to something around 200 (if at
> all). There are a few omap/musb boards setting it to 500.
People haven't paid all that much attention to it. Most likely it
doesn't matter all that much except for OTG hosts.
> The interresting part is davinci of course. So there is comment that says
> |"irlml6401 switches over 1A in under 8 msec"
> and it calls davinci_setup_usb(1000, 8); which writes 255 in the power member
> which musb in turn multiplies by 2. So we have 500mA again ;) I have no idea
> how many ports they have.
Hopefully they will work correctly with these changes regardless.
Alan Stern
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