Hello, Thanks again for the quick feedback. Le Mon, 25 Jul 2011 15:08:53 -0400 (EDT), Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> a écrit : > > I would have expected udev->bus_mA to be 75 mA for the second device, > > so that the 100 mA configuration gets rejected. > > No. The USB stack doesn't enforce any scheme for dividing up power > among a hub's children. All it does is log a warning if the devices > want more power than the total available. Ok. This is also what I understood. > > However, looking at > > http://lxr.free-electrons.com/source/drivers/usb/core/hub.c#L1147, it > > appears that the hub->mA_per_port field is defined to hdev->bus_mA > > (i.e, the power budget). So it means that when we define a power budget > > of 175 mA, the USB core understands that it is 175 mA per port that is > > available, and not 175 mA globally for all ports of the root hub. I > > suspect this explains the behavior I'm seeing. > > > > Unfortunately, this contradicts the computation done in > > hub_power_remaining() at > > http://lxr.free-electrons.com/source/drivers/usb/core/hub.c#L3014, > > which rightfully concludes that 25mA is missing. And this also seems to > > make it difficult to define a global root hub power limit. > > There's no contradiction. If 175 mA total are available, then one way > to apportion it is to plug in a single device requiring 175 mA. In > other words, there really is up to 175 mA available for each port. Hum, I'm sorry, but I still see a contradiction : * On one side, there is a computation that assumes that hdev->bus_mA is a *per-port* current limit (and this computation is used to reject new USB configurations from connecting) ; * On another side, there is a computation that assumes that hdev->bus_mA is a *global hub* current limit (and this computation is used to print a warning in the kernel messages). If I set a global current limit of 175 mA, then two devices of 100 mA shouldn't be allowed to connect. > > Isn't there a contradiction here ? Shouldn't the hdev->bus_mA be > > understood as a global hub power budget, and not a per-port power > > budget ? > > That _is_ how it is interpreted. Well, I don't think so, since by the example above, we have seen that two 100 mA devices (which accounts for 200 mA of power) can connect to a hub limited to 175 mA. > It looks like your real problem is in hcd.c line 2467 (in > usb_add_hcd()): > > rhdev->bus_mA = min(500u, hcd->power_budget); > > We probably shouldn't use the min() operation here, just set > rhdev->bus_mA to hcd->power_budget. However, I'm not sure how the > various host controller drivers set the power_budget field; some of > them may set it to the total power divided by the number of ports > rather than the total power. If they do, they will have to be fixed > when this change is made. This precise instruction is a fairly clear indication that the rhdev->bus_mA value is a per-port limit and not a global hub limit. Otherwise, there wouldn't have been this maximum value of 500mA. Am I missing something ? Thomas -- Thomas Petazzoni, Free Electrons Kernel, drivers, real-time and embedded Linux development, consulting, training and support. http://free-electrons.com -- 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
