Re: usb hcd questions

On Thu, 25 Oct 2007, jidong xiao wrote:

> Regarding "since root-hub status changes are often interrupt-driven"...
> 
> Not quite understand about this,actually I saw in several place of the
> source code, we still set poll_rh to 1,as I understand,usbcore will
> poll for events whenever poll_rh is set. So if HCD often uses
> interrupt to get root hub events, then under what condition,we need to
> use poll?

There are several different cases, depending on the driver.  One common 
example is a remote-wakeup interrupt occurs; when this happens some 
controllers don't also set the root-hub-status-change bit.  Thus we 
have to tell usbcore to poll for status changes after the root hub is 
resumed.

Another example is OHCI arises from the fact that the interrupt
requests are level-triggered instead of edge-triggered.  Once an
interrupt request has been made, status interrupts must be disabled
until the hub driver has turned off the status-change bit.  During that
time interval (until the hub driver does its job), the driver uses
polling.

> Take uhci-hcd for example, note that, poll_rh is set inside
> start_rh/uhci_resume, why the root hub needs to be polled at these
> circumstances?

UHCI is strange.  It generates interrupts for root-hub status change 
events while the root hub is suspended but not while the root hub is 
running!  So when the root hub first is started, and whenever it is 
resumed, the root hub needs to be polled.

Alan Stern


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