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SubjectRE: [PATCH v9 5/6] usb:cdns3 Add Cadence USB3 DRD Driver
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Hi,

Pawel Laszczak <pawell@cadence.com> writes:
>>>>>>Quick question, then: these ISTS registers, are they masked interrupt
>>>>>>status or raw interrupt status?
>>>>>
>>>>> Yes it's masked, but after masking them the new interrupts will not be reported
>>>>> In ISTS registers. Form this reason I can mask only reported interrupt.
>>>>
>>>>and what happens when you unmask the registers? Do they get reported?
>>>
>>> No they are not reported in case of USB_ISTS register.
>>> They should be reported in case EP_ISTS, but I need to test it.
>>
>>okay, please _do_ test and verify the behavior. The description above
>>sounds really surprising to me. Does it really mean that if you mask all
>>USB_ISTS and then disconnect the cable while interrupt is masked, you
>>won't know cable was disconnected?
>
> Yes, exactly.
>
> Initially I've tested it and it's work correct.
> I can even simply write 0 to EP_IEN in hard irq and ~0 in thread handler.
> It's simplest and sufficient way.

okay. Just to be sure I understand correctly. If you mask USB_IEN, then
we would miss a cable disconnect event. Right?

>>>>>>>>> + struct cdns3_aligned_buf *buf, *tmp;
>>>>>>>>> +
>>>>>>>>> + list_for_each_entry_safe(buf, tmp, &priv_dev->aligned_buf_list,
>>>>>>>>> + list) {
>>>>>>>>> + if (!buf->in_use) {
>>>>>>>>> + list_del(&buf->list);
>>>>>>>>> +
>>>>>>>>> + spin_unlock_irqrestore(&priv_dev->lock, flags);
>>>>>>>>
>>>>>>>>creates the possibility of a race condition
>>>>>>> Why? In this place the buf can't be used.
>>>>>>
>>>>>>but you're reenabling interrupts, right?
>>>>>
>>>>> Yes, driver frees not used buffers here.
>>>>> I think that it's the safest place for this purpose.
>>>>
>>>>I guess you missed the point a little. Since you reenable interrupts
>>>>just to free the buffer, you end up creating the possibility for a race
>>>>condition. Specially since you don't mask all interrupt events. The
>>>>moment you reenable interrupts, one of your not-unmasked interrupt
>>>>sources could trigger, then top-half gets scheduled which tries to wake
>>>>up the IRQ thread again and things go boom.
>>>
>>> Ok, I think I understand. So I have 3 options:
>>> 1. Mask the USB_IEN and EP_IEN interrupts, but then I can lost some USB_ISTS
>>> events. It's dangerous options.
>>
>>sure sounds dangerous, but also sounds quite "peculiar" :-)
>>
>>> 2. Remove implementation of handling unaligned buffers and assume that
>>> upper layer will worry about this. What with vendor specific drivers that
>>> can be used by companies and not upstreamed ?
>>> It could be good to have such safety mechanism even if it is not currently used.
>>
>>dunno. It may become dead code that's NEVER used :-)
>>
>>> 3. Delegate this part of code for instance to separate thread that will be called
>>> In free time.
>>
>>Yet another thread? Can't you just run this right before giving back the
>>USB request? So, don't do it from IRQ handler, but from giveback path?
>
> Do you mean in:
> if (request->complete) {
> spin_unlock(&priv_dev->lock);
> if (priv_dev->run_garbage_collector) {
> ....
> }
> usb_gadget_giveback_request(&priv_ep->endpoint,
> request);
> spin_lock(&priv_dev->lock);
> }
> ??

right, you can do it right before giving back the request. Or right
after.

> I ask because this is finally also called from IRQ handler:
>
> cdns3_device_thread_irq_handler
> -> cdns3_check_ep_interrupt_proceed
> -> cdns3_transfer_completed
> -> cdns3_gadget_giveback
> -> usb_gadget_giveback_request

Did you notice that it doesn't reenable interrupts, though?

--
balbi
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