On 2021-08-17 18:53:13 [+0900], Jeaho Hwang wrote:
> hw_ep_prime sometimes fails if irq occurs while it rus on RT kernel.
How/ why does it fail? Which IRQ occurs? Does it also occur without RT
and with threadirqs enabled?
> local_irq_save/restore is added inside the function to gurantee atomicity.
> only effective for preempt_rt since hw_ep_prime is called inside top half
> or spin_lock_irqsave. No effect is expected for standard linux.
How is that helping?
#1
udc_irq() -> isr_tr_complete_handler() -> isr_tr_complete_low ->
_hardware_dequeue() -> reprime_dtd() -> hw_ep_prime()
udc_irq() acquires ci->lock.
#2
ep_queue -> _ep_queue() ->_hardware_enqueue() -> hw_ep_prime()
ep_queue acquires hwep->lock. Which is actually ci->lock.
So if I read this right then hw_ep_prime() may not be interrupted in the
middle of its operation (but preempted) because each path is protected
by the lock.
isr_tr_complete_low() drops hwep->lock and acquires it again so it that
phase another thread may acquire it.
> Signed-off-by: Jeaho Hwang <jhhwang@xxxxxxxxxx>
>
> diff --git a/drivers/usb/chipidea/udc.c b/drivers/usb/chipidea/udc.c
> index 8834ca613721..a624eddb3e22 100644
> --- a/drivers/usb/chipidea/udc.c
> +++ b/drivers/usb/chipidea/udc.c
> @@ -191,22 +191,31 @@ static int hw_ep_get_halt(struct ci_hdrc *ci, int num, int dir)
> static int hw_ep_prime(struct ci_hdrc *ci, int num, int dir, int is_ctrl)
> {
> int n = hw_ep_bit(num, dir);
> + unsigned long flags;
> + int ret = 0;
>
> /* Synchronize before ep prime */
> wmb();
>
> - if (is_ctrl && dir == RX && hw_read(ci, OP_ENDPTSETUPSTAT, BIT(num)))
> + /* irq affects this routine so irq should be disabled on RT.
> + * on standard kernel, irq is already disabled by callers.
The important part is _how_ it is affected. If locking works then
nothing should read/ write the HW register. If the lock is briefly
dropped then another thread _may_ read/ write the registers but not
within this function.
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.
> + */
> + local_irq_save(flags);
> + if (is_ctrl && dir == RX && hw_read(ci, OP_ENDPTSETUPSTAT, BIT(num))) {
> + local_irq_restore(flags);
> return -EAGAIN;
> + }
>
> hw_write(ci, OP_ENDPTPRIME, ~0, BIT(n));
>
> while (hw_read(ci, OP_ENDPTPRIME, BIT(n)))
> cpu_relax();
> if (is_ctrl && dir == RX && hw_read(ci, OP_ENDPTSETUPSTAT, BIT(num)))
> - return -EAGAIN;
> + ret = -EAGAIN;
>
> + local_irq_restore(flags);
> /* status shoult be tested according with manual but it doesn't work */
> - return 0;
> + return ret;
> }
>
> /**
Sebastian
