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SubjectRe: [PATCH v2] usb: chipidea: local_irq_save/restore added for hw_ep_prime
2021년 8월 21일 (토) 오후 2:05, Peter Chen <peter.chen@kernel.org>님이 작성:
>
> On 21-08-20 14:15:55, Jeaho Hwang wrote:
> > 2021년 8월 19일 (목) 오후 5:48, Sebastian Andrzej Siewior
> > <bigeasy@linutronix.de>님이 작성:
> > >
> > > On 2021-08-19 08:50:27 [+0900], Jeaho Hwang wrote:
> > > > Without RT, udc_irq runs as a forced threaded irq handler, so it runs
> > > > without any interruption or preemption. NO similar case is found on
> > > > non-RT.
> > >
> > > I see only a devm_request_irq() so no force-threading here. Booting with
> > > threadirqs would not lead to the problem since commit
> > > 81e2073c175b8 ("genirq: Disable interrupts for force threaded handlers")
> > >
> >
> > I was wrong. udc threaded irq handler allows twd interrupt even on
> > non-RT and with threaded irq.
> > I believed Chen's comment "The function hw_ep_prime is only called at
> > udc_irq which is registered as top-half irq handlers. Why the timer
> > interrupt is occurred when hw_ep_prime is executing?".
>
> Hi Jeaho,
>
> How could you let udc irq as threaded irq? The chipidea interrupt
> is registered using devm_request_irq.
>

HI Peter.

We configured the kernel as "low latency desktop" and added
"threadirqs" inside the cmdline parameter.
Then udc irq handler runs as a thread and shows no suspicious working.
I Hope It will help.

Thanks.

> > We have additional experiments and got the results like below. RNDIS
> > host was Windows.
> >
> > RT, 1ms delay between first ENDPTSETUPSTAT read and priming : error
> > case occurred
> > RT, 1ms delay + irq_save : no error case occurred.
> > non-RT, threaded irq, 1ms delay : no error case occurred even twd
> > fires inside the function execution.
>
> Again, how do you observe it?
>
> Peter
>
> >
> > It doesn't seem to be a timing issue. But irq definitely affects
> > priming on the RT kernel. Do you RT experts have any idea about the
> > causes?
> > If isr_tr_complete_handler fails ep priming it calls _ep_set_halt and
> > goes an infinite loop in hw_ep_set_halt. It was an actual problem we
> > experienced.
> > So we protect irqs inside hw_ep_priming not to make error cases and
> > also add a timeout inside the hw_ep_set_halt loop for a walkaround.
> > The timeout patch is submitted to linux-usb.
> > ( https://marc.info/?l=linux-usb&m=162918269024007&w=2 )
> >
> > We withdrew this patch since we don't know if disabling irq is the
> > best solution to solve the problem and udc would work fine with
> > hw_ep_set_halt walkaround even though hw_ep_prime fails.
> > But we are still trying to find out the cause of this symptom so We'd
> > so appreciate it if RT or USB experts share some ideas or ways to
> > report somewhere. Xilinx doesn't provide any support without their
> > official kernel :(
> >
> > Thanks for the discussion Sebastian.
> >
> > Jeaho Hwang.
> >
> > > …
> > > > > If this function here is sensitive to timing (say the cpu_relax() loop
> > > > > gets interrupt for 1ms) then it has to be documented as such.
> > > >
> > > > The controller sets ENDPTSETUPSTAT register if the host sent a setup packet.
> > > > yes it is a timing problem. I will document that and resubmit again if
> > > > you agree that local_irq_save could help from the timing problem.
> > > >
> > > > Thanks for the advice.
> > >
> > > If it is really a timing issue in the function as you describe below
> > > then disabling interrupts would help and it is indeed an RT only issue.
> > >
> > > So you read OP_ENDPTSETUPSTAT, it is 0, all good.
> > > You write OP_ENDPTPRIME, wait for it to be cleared.
> > > Then you read OP_ENDPTSETUPSTAT again and if it is 0, all good.
> > >
> > > And the TWD interrupt could delay say the second read would read 1 and
> > > it is invalidated. Which looks odd.
> > > However, it is "okay" if the TWD interrupt happens after the second
> > > read? Even if the host sends a setup packet, nothing breaks?
> > > Do you have numbers on how long irq-off section is here? It seems to
> > > depend on how long the HW needs to clear the OP_ENDPTPRIME bits.
> > >
> > > Sebastian
> >
> >
> >
> > --
> > 황재호, Jay Hwang, linux team manager of RTst
> > 010-7242-1593
>
> --
>
> Thanks,
> Peter Chen
>


--
황재호, Jay Hwang, linux team manager of RTst

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