On 8/25/2022 6:30 PM, Thinh Nguyen wrote:
On Tue, Aug 23, 2022, Elson Serrao wrote:
On 8/22/2022 6:01 PM, Thinh Nguyen wrote:
On Thu, Aug 18, 2022 at 11:17:24AM -0700, Elson Serrao wrote:
On 8/16/2022 4:51 PM, Thinh Nguyen wrote:
On 8/16/2022, Elson Serrao wrote:
On 8/12/2022 5:46 PM, Thinh Nguyen wrote:
On 8/11/2022, Thinh Nguyen wrote:
On 8/11/2022, Thinh Nguyen wrote:
On 8/11/2022, Elson Serrao wrote:
On 8/9/2022 6:08 PM, Thinh Nguyen wrote:
<snip>
To summarize the points:
1) The host only arms function remote wakeup if the device is
capable of
remote wakeup (check USB_CONFIG_ATT_WAKEUP in bmAttributes and
hardware
capability)
2) If the device is in suspend, the device can do remote wakeup
(through
LFPS handshake) if the function is armed for remote wakeup (through
SET_FEATURE(FUNC_SUSPEND)).
3) If the link transitions to U0 after the device triggering a remote
wakeup, the device will also send device notification function
wake for
all the interfaces armed with remote wakeup.
4) If the device is not in suspend, the device can send device
notification function wake if it's in U0.
Now, remote wakeup and function wake device notification are 2
separate
operations. We have the usb_gadget_ops->wakeup() for remote wakeup. I
suggested to maybe add
usb_gadget_ops->send_wakeup_notification(gadget,
intf_id) for the device notification. What you did was combining both
operations in usb_gadget_ops->func_wakeup(). That may only work for
point 4) (assuming you fix the U0 check), but not point 3).
Thank you for your feedback and summary. I will rename func_wakeup to
send_wakeup_notification to better align with the approach. The
reason I
have combined remote_wakeup and function wake notification in
usb_gadget_ops->func_wakeup() is because since the implementation
is at
function/composite level it has no knowledge on the link state. So I
have delegated that task to controller driver to handle the
notification
accordingly. That is do a LFPS handshake first if the device is
suspended and then send notification (explained below). But we can
definitely separate this by adding an additional flag in the composite
layer to set the link state based on the gadget suspend callback
called
when U3 SUSPEND interrupt is received. Let me know if you feel
separating the two is a better approach.
The reason I think we need to separate it is because of point 3. As I
note earlier, the spec states that "If remote wake event occurs in
multiple functions, each function shall send a Function Wake
Notification."
But if there's no remote wake event, and the host brought the device up
instead, then the function suspend state is retained.
If we separate these 2 operations, the caller can check whether the
operation went through properly. For example, if the wakeup() is
initiated properly, but the function wake device notification didn't go
through. We would only need to resend the device notification rather
than initiate remote wakeup again.
If we don't have to send device notification for other interfaces, we
can combine the operations here as you did.
I still think it's better to split up the operations. The way you're
handling it now is not clear.
If the func_awake() returns -EAGAIN, I'd expect that the remote wake did
not go through and expect user to retry again. But here it does initiate
remote wake, but it just does not send device notification yet. This is
confusing.
Also, instead of all the function wake handling coming from the function
driver, now we depend on the controller driver to call function resume()
on state change to U0, which will trigger device notification. What
happen if it doesn't call resume(). There's too many dependencies and it
seems fragile.
I think all this can be handled in the function driver. You can fix the
dwc3 wakeup() and poll for U0/ON state rather than RECOVERY state, which
is what it's supposed to poll.
For transitioning from U3 to U0, the current upstream implementation is
to poll for U0 state when dwc3_gadget_wakeup() is called and it is a
blocking call. (this is a common API for both HS and SS)
/* poll until Link State changes to ON */
retries = 20000;
while (retries--) {
reg = dwc3_readl(dwc->regs, DWC3_DSTS);
/* in HS, means ON */
if (DWC3_DSTS_USBLNKST(reg) == DWC3_LINK_STATE_U0)
break;
}
In my experiments I found that certain hosts take longer time to drive
HS resume signalling between the remote wakeup and the resume K and this
time varies across hosts. Hence the above polling timer is not generic
across hosts. So how do we converge on a polling timer value to work
across HS/SS and without blocking the system for a long time?
Can't we take the upper limit of both base on experiment? And it
shouldn't be blocking the whole system.
On the host I was experimenting with, the time it took was around 110ms in
HS case. That would translate to a retry count of about ~181,000 in the
above polling loop. Wouldn't that be a very large value for polling?
And not sure if there are other hosts that take even longer time
We don't want to poll that many times. We shouldn't depend on the delay
of a register read for polling interval. Can't we just sleep in between
interval at a reasonable interval.
Sleeping is not an option as the upper layers (those beyond
function/composite layer) may transmit data with a lock held.
You can use mdelay() if it can't sleep. But if the wait is long, it
should be allowed to sleep.
I ran into below BUG when remote wakeup was triggered via a ping() command
and attempted sleep while polling
[ 88.676789][ T392] BUG: scheduling while atomic
[ 88.900112][ T392] Call trace:
<snip>
[ 88.912760][ T392] __schedule_bug+0x90/0x188
[ 88.917335][ T392] __schedule+0x714/0xb4c
[ 88.930568][ T392] schedule+0x110/0x204
[ 88.943620][ T392] schedule_timeout+0x94/0x134
[ 88.948371][ T392] __dwc3_gadget_wakeup+0x1ac/0x558
[ 88.953558][ T392] dwc3_gadget_wakeup+0x3c/0x8c
[ 88.958388][ T392] usb_gadget_wakeup+0x54/0x1a8
[ 88.963222][ T392] eth_start_xmit+0x130/0x830
[ 88.967876][ T392] xmit_one+0xf0/0x364
[ 88.971913][ T392] sch_direct_xmit+0x188/0x3e4
[ 88.976663][ T392] __dev_xmit_skb+0x480/0x984
[ 88.981319][ T392] __dev_queue_xmit+0x2d4/0x7d8
[ 88.986151][ T392] neigh_resolve_output+0x208/0x2f0
<snip>
The above experiment was done by removing spin_locks if any in the wakeup()
path of function/composite/controller drivers. It is running in atomic
context due to the lock held by linux networking stack/generic packet
scheduler.
So below are the only two approaches I can think of for dwc3_gadget_wakeup()
API
1.)Polling based approach: We poll until the link comes up. But cannot sleep
while polling due to above reasons.
2.)Interrupt based approach (the patches being discussed currently): When a
remote wakeup is triggered enable link state interrupts and return right
away. The function/composite drivers are later notified about the ON event
via resume callback (in a similar way how we notify U3 to composite layer
via suspend callback).
Please let me know if there is any alternate way that you can think of here.
The main issue we're trying to solve is knowing if the host had woken up
and the device notification is sent so that the function driver can
resume().
If we can say that upon usb_gadget_wakeup() successful completion, the
link is in U0/ON, then the function driver can send the wake
notification after and resume(). That is, we're trying to make
usb_gadget_wakeup() synchronous. Whether it's polling or interrupt
based, it's implementation detail.
Unfortunately, the API isn't very clear whether usb_gadget_wakeup() may
sleep or synchronous.
Here are 3 approaches that we can have:
1) Clarify that usb_gadget_wakeup() is synchronous and the link will be
in U0/ON upon successful completion, and clarify whether it can sleep.
Introduce a separate API usb_gadget_send_wake_notification() to send
wake notification separately.
2) Create a new API usb_gadget_function_wakeup() that will combine both
device wakeup and wake notification. The function can sleep,
synchronous, and both wakeup and wake notification are done after the
function completes.
3) Create a new API usb_gadget_function_wakeup() that will combine both
device wakeup and wake notification. The function is asynchronous. We
won't know if the wakeup is successfully sent, but we don't care and
proceed with the function proceed with resume() anyway.
BR,
Thinh
Thank you for your suggestions.
For handling function wakeup (applicable to enhanced super-speed) will
implement a separate API usb_gadget_function_wakeup() that combines
device-wakeup and wake-notification like below in dwc3 driver and
operates synchronously.
dwc3_gadget_func_wakeup()
{
if (link in U3)
Call dwc3_gadget_wakeup()
Poll for U0
If U0 successful, send wake_notification
}
Once the function completes both device wake and func wakeup
notification are done.
For high speed use-cases when usb_gadget_wakeup() is called from
function drivers, considering the possibility of significant delay
associated with remote wakeup, dwc3_gadget_wakeup() should operate
asynchronously.
i.e rely on link status change events rather than sleeping/polling.
Please let me know if there are any concerns. If not will upload new
patch sets with this change and other changes suggested.
Regards
Elson
Some data layers like TCP/IP hold a TX lock while sending data (that
causes a remote wakeup event) and hence this wakeup() may run in atomic
context.
Why hold the lock while waiting for the host to wakeup? The host is
still inactive. Also, the usb_gadget_wakeup() API doesn't specify that
it may run in atomic context.
The lock might be held by upper layers who are unaware/independent of
underlying transport medium. The above TX lock I was referring to was
that held by Linux networking stack. It just pushes out data the same way it
would when USB is active. It is the function/composite layer being aware of
the function suspend would now sense this as a remote wake event and perform
this additional step of bringing out the link from u3 and then sending
device wakeup notification.
In our current upstream implementation of dwc3_gadget_wakeup() API we hold a
spinlock as well. But yeah that can be rectified
Holding a spin_lock for this long is not reasonable. We only need to
lock when setting link recovery request but not while polling for DSTS
and waiting for the link to go up.
BR,
Thinh
static int dwc3_gadget_wakeup(struct usb_gadget *g)
{
struct dwc3 *dwc = gadget_to_dwc(g);
unsigned long flags;
int ret;
spin_lock_irqsave(&dwc->lock, flags);
ret = __dwc3_gadget_wakeup(dwc);
spin_unlock_irqrestore(&dwc->lock, flags);
return ret;
}
To make this generic across hosts, I had switched to interrupt based
approach, enabling link state events and return error value immediately
from the dwc3_gadget_wakeup() API after doing a LFPS handshake. But
yeah, then we have to rely on the resume callback as an indication that
link is transitioned to ON state.
BR,
Thinh
