[PATCH v5 1/2] PCI: mediatek: Clear IRQ status after IRQ dispatched to avoid reentry

[Honghui Zhang]

On Thu, 2018-01-04 at 19:04 +0000, Marc Zyngier wrote:
> On 04/01/18 18:40, Lorenzo Pieralisi wrote:
> > [+Marc]
> > 
> > On Wed, Dec 27, 2017 at 08:59:53AM +0800, honghui.zhang at mediatek.com wrote:
> >> From: Honghui Zhang <honghui.zhang at mediatek.com>
> >>
> >> There maybe a same IRQ reentry scenario after IRQ received in current
> >> IRQ handle flow:
> >> 	EP device		PCIe host driver	EP driver
> >> 1. issue an IRQ
> >> 			2. received IRQ
> >> 			3. clear IRQ status
> >> 			4. dispatch IRQ
> >> 						5. clear IRQ source
> >> The IRQ status was not successfully cleared at step 2 since the IRQ
> >> source was not cleared yet. So the PCIe host driver may receive the
> >> same IRQ after step 5. Then there's an IRQ reentry occurred.
> >> Even worse, if the reentry IRQ was not an IRQ that EP driver expected,
> >> it may not handle the IRQ. Then we may run into the infinite loop from
> >> step 2 to step 4.
> >> Clear the IRQ status after IRQ have been dispatched to avoid the IRQ
> >> reentry.
> >> This patch also fix another INTx IRQ issue by initialize the iterate
> >> before the loop. If an INTx IRQ re-occurred while we are dispatching
> >> the INTx IRQ, then iterate may start from PCI_NUM_INTX + INTX_SHIFT
> >> instead of INTX_SHIFT for the second time entering the
> >> for_each_set_bit_from() loop.
> > 
> > This looks like two different issues that should be fixed with two
> > patches.

Ok, I split this into two patches and figure out a more reasonable
approach by using irq_chip solution.

> > 
> >> Signed-off-by: Honghui Zhang <honghui.zhang at mediatek.com>
> >> Acked-by: Ryder Lee <ryder.lee at mediatek.com>
> >> ---
> >>  drivers/pci/host/pcie-mediatek.c | 11 ++++++-----
> >>  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > For the sake of uniformity, I first want to understand why this
> > driver does not call:
> > 
> > chained_irq_enter/exit()
> > 
> > in the primary handler (mtk_pcie_intr_handler()).
> > 
> > With the GIC as a primary interrupt controller we have not
> > even figured out how current code can actually work without
> > calling the chained_* API.
> > 
> > I want to come up with a consistent handling of IRQ domains for
> > all host bridges and any discrepancy should be explained.
> 
> That's because this driver is a huge hack, see below:
> 
> > 
> >> diff --git a/drivers/pci/host/pcie-mediatek.c b/drivers/pci/host/pcie-mediatek.c
> >> index db93efd..fc29a9a 100644
> >> --- a/drivers/pci/host/pcie-mediatek.c
> >> +++ b/drivers/pci/host/pcie-mediatek.c
> >> @@ -601,15 +601,16 @@ static irqreturn_t mtk_pcie_intr_handler(int irq, void *data)
> 
> This function is not a chained irqchip, but an interrupt handler...
> 
> >>  	struct mtk_pcie_port *port = (struct mtk_pcie_port *)data;
> >>  	unsigned long status;
> >>  	u32 virq;
> >> -	u32 bit = INTX_SHIFT;
> >> +	u32 bit;
> >>  
> >>  	while ((status = readl(port->base + PCIE_INT_STATUS)) & INTX_MASK) {
> >> +		bit = INTX_SHIFT;
> >>  		for_each_set_bit_from(bit, &status, PCI_NUM_INTX + INTX_SHIFT) {
> >> -			/* Clear the INTx */
> >> -			writel(1 << bit, port->base + PCIE_INT_STATUS);
> >>  			virq = irq_find_mapping(port->irq_domain,
> >>  						bit - INTX_SHIFT);
> >>  			generic_handle_irq(virq);
> 
> and nonetheless, this calls into generic_handle_irq(). That's a complete
> violation of the interrupt layering. Maybe there is a good reason for
> it, but I'd like to know which one.
> 
> Which means that all of the ack/mask has to be done outside of the
> irqchip framework too... Disgusting.
> 
> >> +			/* Clear the INTx */
> >> +			writel(1 << bit, port->base + PCIE_INT_STATUS);
> > 
> > I think that these masking/acking should actually be done through
> > the irq_chip hooks (see for instance pci-ftpci100.c) - that would
> > make this kind of bugs much easier to prevent (because the IRQ
> > layer does the sequencing for you).
> 
> +1.
> 

Thanks for your advice, I need to do some homework to have a better
understanding of the irq_chip approach.

> > Marc (CC'ed) has a more comprehensive view on this than me - I would
> > like to get to a point where all host bridges uses a consistent
> > approach for chained IRQ handling and I hope this bug fix can be
> > a starting point.
> 
> +1 again. We definitely need to come up with some form of common
> approach for all these host drivers, and maybe turn that into a library...
> 

Well, this is beyond my knowledge now, I guess I can figure out how to
using irq_chip for the first step, then I may following this "common
approach" after we have a solution for that?

thanks.
> Thanks,
> 
> 	M.