linux-kernel - RE: [PATCH v9 5/6] usb:cdns3 Add Cadence USB3 DRD Driver

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Date:   Mon, 12 Aug 2019 12:45:30 +0300
From:   Felipe Balbi <felipe.balbi@...ux.intel.com>
To:     Pawel Laszczak <pawell@...ence.com>,
        "devicetree\@vger.kernel.org" <devicetree@...r.kernel.org>
Cc:     "gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
        "linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
        "hdegoede\@redhat.com" <hdegoede@...hat.com>,
        "heikki.krogerus\@linux.intel.com" <heikki.krogerus@...ux.intel.com>,
        "robh+dt\@kernel.org" <robh+dt@...nel.org>,
        "rogerq\@ti.com" <rogerq@...com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "jbergsagel\@ti.com" <jbergsagel@...com>,
        "nsekhar\@ti.com" <nsekhar@...com>, "nm\@ti.com" <nm@...com>,
        Suresh Punnoose <sureshp@...ence.com>,
        "peter.chen\@nxp.com" <peter.chen@....com>,
        Jayshri Dajiram Pawar <jpawar@...ence.com>,
        Rahul Kumar <kurahul@...ence.com>
Subject: RE: [PATCH v9 5/6] usb:cdns3 Add Cadence USB3 DRD Driver


Hi,

Pawel Laszczak <pawell@...ence.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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