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

[Felipe Balbi]

Hi,

Pawel Laszczak <pawell@xxxxxxxxxxx> writes:
>>> I have such situation in which one interrupt line is shared with ehci and cdns3 driver.
>>> In such case this function returns error code.
>>
>>which function returns error code?
>
> devm_request_threaded_irq, of course if I set IRQF_SHARED | IRQF_ONESHOT.
> As I remember it was EBUSY error.

oh, right. That's probably because the handlers must agree on IRQ flags.

>>> So probably I will need to mask only the reported interrupts.
>>
>>you should mask all interrupts from your device, otherwise you top-halt
>>may still end up reentrant.
>>
>>> I can't mask all interrupt using controller register because I can miss some of them.
>>
>>why would you miss them? They would be left in the register until you
>>unmask them and the line is raised again.
>
> I consult this with author of controller. 
> We have:
> USB_IEN  and USB_ISTS for  generic interrupts
> EP_IEN and EP_ISTS for endpoint interrupts 
>
> Both these group works different.
> For endpoint I can disable all interrupt and I don't miss any of them. 
> So it's normal behavior.
>
> But USB_ISTS work little different. If we mask all interrupt in USB_IEN
> then when new event occurs the EP_ISTS will not be updated. 

wait a minute. When you mask USB_ISTS, then EP_ISTS isn't updated? Is
this a quirk on the controller or a design choice?

> It's not standard and not expected behavior but it works in this way. 

Yeah, sounds rather odd.

>>>>>>> +	/* check USB device interrupt */
>>>>>>> +	reg = readl(&priv_dev->regs->usb_ists);
>>>>>>> +
>>>>>>> +	if (reg) {
>>>>>>> +		writel(reg, &priv_dev->regs->usb_ists);
>>>>>>> +		cdns3_check_usb_interrupt_proceed(priv_dev, reg);
>>>>>>> +		ret = IRQ_HANDLED;
>>>>>>
>>>>>>now, because you _don't_ mask this interrupt, you're gonna have
>>>>>>issues. Say we actually get both device and endpoint interrupts while
>>>>>>the thread is already running with previous endpoint interrupts. Now
>>>>>>we're gonna reenter the top half, because device interrupts are *not*
>>>>>>masked, which will read usb_ists and handle it here.
>>>>>
>>>>> Endpoint interrupts are masked in cdns3_device_irq_handler and stay masked
>>>>> until they are not handled in threaded handler.
>>>>
>>>>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?

>>>>>>> +		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?

-- 
balbi

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