Re: USB devices fail unnecessarily on unpowered hubs

Re: USB devices fail unnecessarily on unpowered hubs

[Robert Hancock]

Alan Stern wrote:
>>
>> (Goes away and pats all his 240V plugpacks which are happily working off 110V).
> 
> They probably will.  The question is, how far out-of-spec should the 
> kernel allow by default?  200% is likely to be too far (your plugpacks 
> notwithstanding).

I would say by default the kernel should not allow any out-of-spec power 
condition. Some devices might decide to go into over-current mode if a 
device draws too much power, taking down other devices on the USB bus. 
This sort of behavior should only be enabled if the user explicitly 
chooses to live dangerously.

Especially given the fact that Windows enforces the USB power supply 
limits strictly, and that's where all the hardware is best tested..

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

Re: USB devices fail unnecessarily on unpowered hubs

[Robert Hancock]

Lee Dowling wrote:
> This, of course, doesn't deal with outside cases.
> 
> It's common knowledge that a lot of equipment is running out of spec all 
> the time because of cheap components, bad BIOS's etc.  As an example, my 
> Asus L4500R laptop (with the latest ASUS BIOS) ALWAYS shows 
> "over-current" under Linux on all *internal* USB ports the second 
> ANYTHING is plugged in (and I have nearly 50 different USB devices of 
> different types, manufacturers and quality).
> 
> The suggestion to simply stop over-current ports from working would 
> immediately disable all USB ports, including any powered hubs that I 
> plug into them, I assume.  I can't update the BIOS any further to stop 
> this and if I could I doubt it would solve the problem (it looks like 
> cheap hardware to me).  Therefore, you've just removed all my perfectly 
> functional USB capability because the best BIOS I can use reports an 
> incorrect error (hey, what's new?).

I don't think you assume correctly. Overcurrent indication from the hub 
is supposed to mean that something on the hub is shorted or pulling too 
much current. This discussion is about the kernel refusing to enable 
devices when they would consume more power than the hub they are 
connected to can provide. Regardless of what you do with plugging in 
devices, overcurrent isn't supposed to happen unless something is broken.

> 
> Windows XP, incidentally, runs flawlessly with all USB devices without 
> power warnings on this laptop.  This may well be fixable somewhere else, 
> it may even be a bug in the internal USB code for my laptop which may be 
> help in hunting such bugs down.

Probably either XP ignores the overcurrent indication, or else something 
   in Linux is interacting badly with the controller and causing this 
indication. XP definitely DOES enforce USB power budgeting.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

Re: USB devices fail unnecessarily on unpowered hubs

[Lee Dowling]

This, of course, doesn't deal with outside cases.

It's common knowledge that a lot of equipment is running out of spec all  
the time because of cheap components, bad BIOS's etc.  As an example, my  
Asus L4500R laptop (with the latest ASUS BIOS) ALWAYS shows "over-current"  
under Linux on all *internal* USB ports the second ANYTHING is plugged in  
(and I have nearly 50 different USB devices of different types,  
manufacturers and quality).

The suggestion to simply stop over-current ports from working would  
immediately disable all USB ports, including any powered hubs that I plug  
into them, I assume.  I can't update the BIOS any further to stop this and  
if I could I doubt it would solve the problem (it looks like cheap  
hardware to me).  Therefore, you've just removed all my perfectly  
functional USB capability because the best BIOS I can use reports an  
incorrect error (hey, what's new?).

Windows XP, incidentally, runs flawlessly with all USB devices without  
power warnings on this laptop.  This may well be fixable somewhere else,  
it may even be a bug in the internal USB code for my laptop which may be  
help in hunting such bugs down.  However, anything like this should be  
optional and not with some convoluted command-line echo, but by as simple  
binary switch accesible to userspace.  I know what I'm doing, if I choose  
to ignore the error, that's my problem.  The fact is, if I don't ignore  
this particular error, my laptop loses all USB functionality.  Taint my  
kernel if you want (that's what the new userspace taint is for, is it  
not?) but I need to use the USB ports that I've paid a lot of money to  
have and that DO work perfectly if given a nudge.

Spec's are lovely and all, but we all know that if real hardware confirmed  
to the spec's all the time that the Linux kernel would be about half it's  
current size.

Lee Dowling
ICT Technician

Re: [linux-usb-devel] USB devices fail unnecessarily on unpowered hubs

[Alan Stern]

On Thu, 1 Jun 2006, David Liontooth wrote:

> What are the reasons not to do this? What happens if a USB stick is
> underpowered to one unit? Nothing? Slower transmission? Data loss? Flash
> memory destruction? If it's just speed, it's a price well worth paying.

I do wish people would read the earlier messages in this thread before 
posting.  Go back and look at this one:

http://marc.theaimsgroup.com/?l=linux-kernel&m=114918113822427&w=2

Trying to draw too much current from an unpowered hub can and does cause 
data loss.

Alan Stern

Re: USB devices fail unnecessarily on unpowered hubs

[David Liontooth]

Alan Stern wrote:
> Trying to draw too much current from an unpowered hub can and does cause 
> data loss.
>   
I consider this issue closed; thank you for looking at it. The
workaround is reasonably simple for the common situation of mounting a
USB stick on a keyboard, perhaps with a mouse attached in a second port:

NUM="$(dmesg | tail -n 3 | grep -r 'new full speed USB device' | cut -b
5-9)"
echo -n 1 >/sys/bus/usb/devices/$NUM/bConfigurationValue

with the understanding this breaks USB power rules and can lead to data
loss.

Dave

Re: USB devices fail unnecessarily on unpowered hubs

[Robert Hancock]

linux-os (Dick Johnson) wrote:
> Many, most, perhaps all such devices don't take more power when they
> are "enabled". Everything is already running and sucking up maximum
> current when you plug it in! If the motherboard didn't smoke when
> the device was plugged in, you might just as well let the user use
> it! Perhaps a ** WARNING ** message somewhere, but by golly, they
> got it running or else you wouldn't be able to read its parameters.

Wrong.. USB devices are not allowed to draw more than some amount (100 
mA I think) of power before the host tells it that it is allowed to 
switch into full-power mode. Any device that doesn't do this doesn't 
comply with the USB specs. Therefore it is very possible to connect a 
device and discover that it can't be enabled.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

Re: USB devices fail unnecessarily on unpowered hubs

[Randy.Dunlap]

On Thu, 01 Jun 2006 17:35:22 -0600 Robert Hancock wrote:

> linux-os (Dick Johnson) wrote:
> > Many, most, perhaps all such devices don't take more power when they
> > are "enabled". Everything is already running and sucking up maximum
> > current when you plug it in! If the motherboard didn't smoke when
> > the device was plugged in, you might just as well let the user use
> > it! Perhaps a ** WARNING ** message somewhere, but by golly, they
> > got it running or else you wouldn't be able to read its parameters.
> 
> Wrong.. USB devices are not allowed to draw more than some amount (100 
> mA I think) of power before the host tells it that it is allowed to 
> switch into full-power mode. Any device that doesn't do this doesn't 
> comply with the USB specs. Therefore it is very possible to connect a 
> device and discover that it can't be enabled.

so does Thunderbird on Windows have absolutely no Reply-to-All
and no References or In-Reply-To capabilities?

---
~Randy

USB devices fail unnecessarily on unpowered hubs

[David Liontooth]

Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
hubs. Alan Stern explains,

"The idea is that the kernel now keeps track of USB power budgets.  When a 
bus-powered device requires more current than its upstream hub is capable 
of providing, the kernel will not configure it.

Computers' USB ports are capable of providing a full 500 mA, so devices
plugged directly into the computer will work okay.  However unpowered hubs
can provide only 100 mA to each port.  Some devices require (or claim they
require) more current than that.  As a result, they don't get configured
when plugged into an unpowered hub."

http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg43480.html

This is generating a lot of grief and appears to be unnecessarily
strict. Common USB sticks with a MaxPower value just above 100mA, for
instance, typically work fine on unpowered hubs supplying 100mA.

Is a more user-friendly solution possible? Could the shortfall
information be passed to udev, which would allow rules to be written per
device?

David

Re: USB devices fail unnecessarily on unpowered hubs

[Pavel Machek]

Hi!

> Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
> hubs. Alan Stern explains,
> 
> "The idea is that the kernel now keeps track of USB power budgets.  When a 
> bus-powered device requires more current than its upstream hub is capable 
> of providing, the kernel will not configure it.
> 
> Computers' USB ports are capable of providing a full 500 mA, so devices
> plugged directly into the computer will work okay.  However unpowered hubs
> can provide only 100 mA to each port.  Some devices require (or claim they
> require) more current than that.  As a result, they don't get configured
> when plugged into an unpowered hub."

Actually I have exactly opposite problem: my computer (spitz) can't
supply full 500mA on its root hub, and linux tries to power up
'hungry' devices, anyway, leading to very weird behaviour.

-- 
Thanks for all the (sleeping) penguins.

Re: USB devices fail unnecessarily on unpowered hubs

[Oliver Neukum]

Am Dienstag, 30. Mai 2006 22:01 schrieb Pavel Machek:
> Hi!
> 
> > Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
> > hubs. Alan Stern explains,
> > 
> > "The idea is that the kernel now keeps track of USB power budgets.  When a 
> > bus-powered device requires more current than its upstream hub is capable 
> > of providing, the kernel will not configure it.
> > 
> > Computers' USB ports are capable of providing a full 500 mA, so devices
> > plugged directly into the computer will work okay.  However unpowered hubs
> > can provide only 100 mA to each port.  Some devices require (or claim they
> > require) more current than that.  As a result, they don't get configured
> > when plugged into an unpowered hub."
> 
> Actually I have exactly opposite problem: my computer (spitz) can't
> supply full 500mA on its root hub, and linux tries to power up
> 'hungry' devices, anyway, leading to very weird behaviour.


You could lower the obvious values in this code from drivers/usb/core/hub.c

	if (hdev == hdev->bus->root_hub) {
		if (hdev->bus_mA == 0 || hdev->bus_mA >= 500)
			hub->mA_per_port = 500;
		else {
			hub->mA_per_port = hdev->bus_mA;
			hub->limited_power = 1;
		}

If that does the job we need to somehow inherit the power supply maximum from
PCI when we allocate the root hub's device structure.

	Regards
		Oliver

Re: USB devices fail unnecessarily on unpowered hubs

[Andrew Morton]

On Thu, 01 Jun 2006 02:18:20 -0700
David Liontooth <liontooth@cogweb.net> wrote:

> Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
> hubs. Alan Stern explains,
> 
> "The idea is that the kernel now keeps track of USB power budgets.  When a 
> bus-powered device requires more current than its upstream hub is capable 
> of providing, the kernel will not configure it.
> 
> Computers' USB ports are capable of providing a full 500 mA, so devices
> plugged directly into the computer will work okay.  However unpowered hubs
> can provide only 100 mA to each port.  Some devices require (or claim they
> require) more current than that.  As a result, they don't get configured
> when plugged into an unpowered hub."
> 
> http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg43480.html
> 
> This is generating a lot of grief and appears to be unnecessarily
> strict. Common USB sticks with a MaxPower value just above 100mA, for
> instance, typically work fine on unpowered hubs supplying 100mA.
> 
> Is a more user-friendly solution possible? Could the shortfall
> information be passed to udev, which would allow rules to be written per
> device?
> 

(added linux-usb cc)

Yes, it sounds like we're being non-real-worldly here.  This change
apparently broke things.  Did it actually fix anything as well?

Re: USB devices fail unnecessarily on unpowered hubs

[Daniel Drake]

Andrew Morton wrote:
> (added linux-usb cc)
> 
> Yes, it sounds like we're being non-real-worldly here.  This change
> apparently broke things.  Did it actually fix anything as well?

Gentoo recieved several reports of this. It appears that certain vendors 
are worse than others (Verbatim flash drives are a common culprit).

Some users tested and found that Windows has the same behaviour - it 
rejects these devices on unpowered hubs, and pops up a warning that not 
enough power is available.

I added a printk to point out when configurations are rejected due to 
power issues, this has been merged into Greg's tree. It's far from 
ideal, but better than silent failure...

Daniel

Re: USB devices fail unnecessarily on unpowered hubs

[Alan Stern]

On Thu, 1 Jun 2006, Andrew Morton wrote:

> On Thu, 01 Jun 2006 02:18:20 -0700
> David Liontooth <liontooth@cogweb.net> wrote:
> 
> > Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
> > hubs. Alan Stern explains,
> > 
> > "The idea is that the kernel now keeps track of USB power budgets.  When a 
> > bus-powered device requires more current than its upstream hub is capable 
> > of providing, the kernel will not configure it.
> > 
> > Computers' USB ports are capable of providing a full 500 mA, so devices
> > plugged directly into the computer will work okay.  However unpowered hubs
> > can provide only 100 mA to each port.  Some devices require (or claim they
> > require) more current than that.  As a result, they don't get configured
> > when plugged into an unpowered hub."
> > 
> > http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg43480.html
> > 
> > This is generating a lot of grief and appears to be unnecessarily
> > strict. Common USB sticks with a MaxPower value just above 100mA, for
> > instance, typically work fine on unpowered hubs supplying 100mA.
> > 
> > Is a more user-friendly solution possible? Could the shortfall
> > information be passed to udev, which would allow rules to be written per
> > device?

I'm not sure whether we create a udev event when a new USB device is
connected.  If we don't, we certainly could.  Although this event wouldn't
contain information about the power budget shortfall, it would contain
vendor and product IDs.  These would be sufficiently specific for a custom
udev script to install the desired configuration.

> Yes, it sounds like we're being non-real-worldly here.  This change
> apparently broke things.  Did it actually fix anything as well?

Yes.  At least, I think so.  The change directly addresses a complaint 
filed here:

http://marc.theaimsgroup.com/?l=linux-usb-users&m=112438431718562&w=2

I haven't checked back with the original poster to make sure that his
problem has been solved, but presumably it has been.

As an alternative, we could allow an "over-budget window" of say 10%.  
Configurations that exceed the power budget by less than that amount would
still be accepted.  I don't know whether this would be enough of a help,
however.  I've heard of devices that claim to require 200 mA, for
instance.  It just doesn't seem right to enable them when the upstream hub
can only provide 100 mA.

Alan Stern

Re: USB devices fail unnecessarily on unpowered hubs

[linux-os (Dick Johnson)]

On Thu, 1 Jun 2006, Alan Stern wrote:

> On Thu, 1 Jun 2006, Andrew Morton wrote:
>
>> On Thu, 01 Jun 2006 02:18:20 -0700
>> David Liontooth <liontooth@cogweb.net> wrote:
>>
>>> Starting with 2.6.16, some USB devices fail unnecessarily on unpowered
>>> hubs. Alan Stern explains,
>>>
>>> "The idea is that the kernel now keeps track of USB power budgets.  When a
>>> bus-powered device requires more current than its upstream hub is capable
>>> of providing, the kernel will not configure it.
>>>
>>> Computers' USB ports are capable of providing a full 500 mA, so devices
>>> plugged directly into the computer will work okay.  However unpowered hubs
>>> can provide only 100 mA to each port.  Some devices require (or claim they
>>> require) more current than that.  As a result, they don't get configured
>>> when plugged into an unpowered hub."
>>>
>>> http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg43480.html
>>>
>>> This is generating a lot of grief and appears to be unnecessarily
>>> strict. Common USB sticks with a MaxPower value just above 100mA, for
>>> instance, typically work fine on unpowered hubs supplying 100mA.
>>>
>>> Is a more user-friendly solution possible? Could the shortfall
>>> information be passed to udev, which would allow rules to be written per
>>> device?
>
> I'm not sure whether we create a udev event when a new USB device is
> connected.  If we don't, we certainly could.  Although this event wouldn't
> contain information about the power budget shortfall, it would contain
> vendor and product IDs.  These would be sufficiently specific for a custom
> udev script to install the desired configuration.
>
>> Yes, it sounds like we're being non-real-worldly here.  This change
>> apparently broke things.  Did it actually fix anything as well?
>
> Yes.  At least, I think so.  The change directly addresses a complaint
> filed here:
>
> http://marc.theaimsgroup.com/?l=linux-usb-users&m=112438431718562&w=2
>
> I haven't checked back with the original poster to make sure that his
> problem has been solved, but presumably it has been.
>
> As an alternative, we could allow an "over-budget window" of say 10%.
> Configurations that exceed the power budget by less than that amount would
> still be accepted.  I don't know whether this would be enough of a help,
> however.  I've heard of devices that claim to require 200 mA, for
> instance.  It just doesn't seem right to enable them when the upstream hub
> can only provide 100 mA.
>
> Alan Stern
>

Many, most, perhaps all such devices don't take more power when they
are "enabled". Everything is already running and sucking up maximum
current when you plug it in! If the motherboard didn't smoke when
the device was plugged in, you might just as well let the user use
it! Perhaps a ** WARNING ** message somewhere, but by golly, they
got it running or else you wouldn't be able to read its parameters.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.73 BogoMips).
New book: http://www.AbominableFirebug.com/
_
\x1a\x04

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Thank you.

Re: USB devices fail unnecessarily on unpowered hubs

[Lennart Sorensen]

On Thu, Jun 01, 2006 at 11:09:46AM -0400, linux-os (Dick Johnson) wrote:
> Many, most, perhaps all such devices don't take more power when they
> are "enabled". Everything is already running and sucking up maximum
> current when you plug it in! If the motherboard didn't smoke when
> the device was plugged in, you might just as well let the user use
> it! Perhaps a ** WARNING ** message somewhere, but by golly, they
> got it running or else you wouldn't be able to read its parameters.

I imagine something like a harddisk might use more power while
reading/writing than when it is just spinning.  It might even start
powered down until sent some command that causes it to spin up.

A scanner certainly uses more power with the scanner light on than with
it off, and it starts out off until it is in use on most scanners.  Of
course I have never seen a usb powered scanner, so it doesn't seem to
matter.

Len Sorensen

Re: USB devices fail unnecessarily on unpowered hubs

[Dagfinn Ilmari Mannsåker]

Lennart Sorensen <lsorense@csclub.uwaterloo.ca> writes:

> A scanner certainly uses more power with the scanner light on than with
> it off, and it starts out off until it is in use on most scanners.  Of
> course I have never seen a usb powered scanner, so it doesn't seem to
> matter.

Oh, they've been around for years. The CanoScan LiDE 25 is an example:
<http://www.usa.canon.com/consumer/controller?act=ModelTechSpecsAct&fcategoryid=119&modelid=11463>

-- 
ilmari
"A disappointingly low fraction of the human race is,
 at any given time, on fire." - Stig Sandbeck Mathisen

Re: USB devices fail unnecessarily on unpowered hubs

[Oliver Neukum]

Am Donnerstag, 1. Juni 2006 17:09 schrieb linux-os (Dick Johnson):
> Many, most, perhaps all such devices don't take more power when they
> are "enabled". Everything is already running and sucking up maximum
> current when you plug it in! If the motherboard didn't smoke when

If they do, they are violating the spec. A device in the unconfigured (state 0)
state must not draw more than 100mA.

	Regards
		Oliver

Re: USB devices fail unnecessarily on unpowered hubs

[linux-os (Dick Johnson)]

On Thu, 1 Jun 2006, Oliver Neukum wrote:

> Am Donnerstag, 1. Juni 2006 17:09 schrieb linux-os (Dick Johnson):
>> Many, most, perhaps all such devices don't take more power when they
>> are "enabled". Everything is already running and sucking up maximum
>> current when you plug it in! If the motherboard didn't smoke when
>
> If they do, they are violating the spec. A device in the unconfigured (state 0)
> state must not draw more than 100mA.
>
> 	Regards
> 		Oliver
>

Hmmm, the USB-IF recommends 100 mA per port, not requires.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.4 on an i686 machine (5592.73 BogoMips).
New book: http://www.AbominableFirebug.com/
_
\x1a\x04

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The information transmitted in this message is confidential and may be privileged.  Any review, retransmission, dissemination, or other use of this information by persons or entities other than the intended recipient is prohibited.  If you are not the intended recipient, please notify Analogic Corporation immediately - by replying to this message or by sending an email to DeliveryErrors@analogic.com - and destroy all copies of this information, including any attachments, without reading or disclosing them.

Thank you.

Re: USB devices fail unnecessarily on unpowered hubs

[Alan Stern]

On Thu, 1 Jun 2006, linux-os (Dick Johnson) wrote:

> >> Yes, it sounds like we're being non-real-worldly here.  This change
> >> apparently broke things.  Did it actually fix anything as well?
> >
> > Yes.  At least, I think so.  The change directly addresses a complaint
> > filed here:
> >
> > http://marc.theaimsgroup.com/?l=linux-usb-users&m=112438431718562&w=2

> Many, most, perhaps all such devices don't take more power when they
> are "enabled". Everything is already running and sucking up maximum
> current when you plug it in! If the motherboard didn't smoke when
> the device was plugged in, you might just as well let the user use
> it! Perhaps a ** WARNING ** message somewhere, but by golly, they
> got it running or else you wouldn't be able to read its parameters.

Looks like you didn't bother to read that complaint and the follow-up
messages.  Robert Marquardt's device has two configurations, one using 100
mA and the other using 500 mA.  Before my patch, Linux would always
install the high-power config -- even if the device was behind a
bus-powered hub.  According to Robert:

	This can trigger the overcurrent protection of a bus powered 
	hub which usually then switches off completely dragging down
	three other innocent devices.

	Please tell me that Linux kernel programmers are not that idiotic.

I'll avoid speculations about which kernel programmers are or are not 
idiotic...

Alan Stern

Re: USB devices fail unnecessarily on unpowered hubs

[Andrew Morton]

On Thu, 1 Jun 2006 10:58:43 -0400 (EDT)
Alan Stern <stern@rowland.harvard.edu> wrote:

> As an alternative, we could allow an "over-budget window" of say 10%.  

That, plus we should provide a suitable i-know-what-im-doing user override,
with the appropriate warnings, as well as a printk which directs users to
this option when we decided to disable their device.

> Configurations that exceed the power budget by less than that amount would
> still be accepted.  I don't know whether this would be enough of a help,
> however.  I've heard of devices that claim to require 200 mA, for
> instance.  It just doesn't seem right to enable them when the upstream hub
> can only provide 100 mA.

The power supply spec is a conservative minimum, whereas the device spec is
a worst-case maximum.  One would expect a lot of devices will work OK when
run "out of spec".

(Goes away and pats all his 240V plugpacks which are happily working off 110V).

Re: USB devices fail unnecessarily on unpowered hubs

[Alan Stern]

On Thu, 1 Jun 2006, Andrew Morton wrote:

> On Thu, 1 Jun 2006 10:58:43 -0400 (EDT)
> Alan Stern <stern@rowland.harvard.edu> wrote:
> 
> > As an alternative, we could allow an "over-budget window" of say 10%.  
> 
> That, plus we should provide a suitable i-know-what-im-doing user override,
> with the appropriate warnings, as well as a printk which directs users to
> this option when we decided to disable their device.

The user override already exists, as do the appropriate warnings.  Daniel 
Drake just added a printk explaining why the device was not configured, 
although it doesn't tell how to configure the device by hand.  Perhaps a 
FAQ entry at www.linux-usb.org would be appropriate.

> The power supply spec is a conservative minimum, whereas the device spec is
> a worst-case maximum.  One would expect a lot of devices will work OK when
> run "out of spec".
> 
> (Goes away and pats all his 240V plugpacks which are happily working off 110V).

They probably will.  The question is, how far out-of-spec should the 
kernel allow by default?  200% is likely to be too far (your plugpacks 
notwithstanding).

Alan Stern

Re: USB devices fail unnecessarily on unpowered hubs

[Mark Lord]

Andrew Morton wrote:
> On Thu, 01 Jun 2006 02:18:20 -0700
> ..
>> This is generating a lot of grief and appears to be unnecessarily
>> strict. Common USB sticks with a MaxPower value just above 100mA, for
>> instance, typically work fine on unpowered hubs supplying 100mA.
>>
>> Is a more user-friendly solution possible? Could the shortfall
>> information be passed to udev, which would allow rules to be written per
>> device?
..
> Yes, it sounds like we're being non-real-worldly here.  This change
> apparently broke things.  Did it actually fix anything as well?

I think a far more sensible approach would be to just ensure that the
total current draw for the (unpowered) hub and all connected devices,
stays below the 500mA allowed.  So a 200mA device could coexist with
a 100mA device on a hub which itself steals 100mA.

Cheers

Re: USB devices fail unnecessarily on unpowered hubs

[Alan Stern]

On Thu, 1 Jun 2006, linux-os (Dick Johnson) wrote:

> > If they do, they are violating the spec. A device in the unconfigured (state 0)
> > state must not draw more than 100mA.
...
> Hmmm, the USB-IF recommends 100 mA per port, not requires.

See section 7.2.1 of the USB 2.0 specification (p. 177):

	Devices must also ensure that the maximum operating current drawn 
	by a device is one unit load, until configured.

Note that a unit load is defined as to be 100 mA.  This is a requirement, 
not a recommendation.


On Thu, 1 Jun 2006, Mark Lord wrote:

> I think a far more sensible approach would be to just ensure that the
> total current draw for the (unpowered) hub and all connected devices,
> stays below the 500mA allowed.  So a 200mA device could coexist with
> a 100mA device on a hub which itself steals 100mA.

On that same page the specification says:

	Bus-powered hubs: Draw all of their power for any internal
	functions and downstream facing ports from VBUS on the hub s
	upstream facing port.  Bus-powered hubs may only draw up to one
	unit load upon power-up and five unit loads after configuration.
	The configuration power is split between allocations to the hub,
	any non-removable functions and the external ports. External ports
	in a bus-powered hub can supply only one unit load per port
	regardless of the current draw on the other ports of that hub.

This clearly states that a bus-powered hub cannot supply 200 mA on one
port, even if another port is unoccupied.

Alan Stern