Re: [v9,5/7] PCI: mediatek-gen3: Add MSI support

["Pali Rohár" <pali@kernel.org>]

On Saturday 27 March 2021 19:44:30 Marc Zyngier wrote:
> On Sat, 27 Mar 2021 19:28:37 +0000,
> Pali Rohár <pali@kernel.org> wrote:
> > 
> > On Wednesday 24 March 2021 11:05:08 Jianjun Wang wrote:
> > > +static void mtk_pcie_msi_handler(struct mtk_pcie_port *port, int set_idx)
> > > +{
> > > +	struct mtk_msi_set *msi_set = &port->msi_sets[set_idx];
> > > +	unsigned long msi_enable, msi_status;
> > > +	unsigned int virq;
> > > +	irq_hw_number_t bit, hwirq;
> > > +
> > > +	msi_enable = readl_relaxed(msi_set->base + PCIE_MSI_SET_ENABLE_OFFSET);
> > > +
> > > +	do {
> > > +		msi_status = readl_relaxed(msi_set->base +
> > > +					   PCIE_MSI_SET_STATUS_OFFSET);
> > > +		msi_status &= msi_enable;
> > > +		if (!msi_status)
> > > +			break;
> > > +
> > > +		for_each_set_bit(bit, &msi_status, PCIE_MSI_IRQS_PER_SET) {
> > > +			hwirq = bit + set_idx * PCIE_MSI_IRQS_PER_SET;
> > > +			virq = irq_find_mapping(port->msi_bottom_domain, hwirq);
> > > +			generic_handle_irq(virq);
> > > +		}
> > > +	} while (true);
> > 
> > Hello!
> > 
> > Just a question, cannot this while-loop cause block of processing other
> > interrupts?
> 
> This is a level interrupt. You don't have much choice but to handle it
> immediately, although an alternative would be to mask it and deal with
> it in a thread. And since Linux doesn't deal with interrupt priority,
> a screaming interrupt is never a good thing.

I see. Something like "interrupt priority" (which does not exist?) would
be needed to handle it.

> > I have done tests with different HW (aardvark) but with same while(true)
> > loop logic. One XHCI PCIe controller was sending MSI interrupts too fast
> > and interrupt handler with this while(true) logic was in infinite loop.
> > During one IRQ it was calling infinite many times generic_handle_irq()
> > as HW was feeding new and new MSI hwirq into status register.
> 
> Define "too fast".

Fast - next interrupt comes prior checking if while(true)-loop should stop.

> If something in the system is able to program the
> XHCI device in such a way that it causes a screaming interrupt, that's
> the place to look for problems, and probably not in the interrupt
> handling itself, which does what it is supposed to do.
> 
> > But this is different HW, so it can have different behavior and does not
> > have to cause above issue.
> > 
> > I have just spotted same code pattern for processing MSI interrupts...
> 
> This is a common pattern that you will find in pretty much any
> interrupt handling/demuxing, and is done this way when the cost of
> taking the exception is high compared to that of handling it.

And would not help if while(true)-loop is replaced by loop with upper
limit of iterations? Or just call only one iteration?

> Which is pretty much any of the badly designed, level-driving,
> DW-inspired, sorry excuse for MSI implementations that are popular on
> low-end ARM SoCs.

Ok. So thank you for information!

> Thanks,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.