linux-kernel - Re: [PATCH v4 1/4] PCI: X-Gene: Add the APM X-Gene v1 PCIe MSI/MSIX termination driver

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Date:	Sun, 19 Apr 2015 21:55:41 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	Duc Dang <dhdang@....com>, Feng Kan <fkan@....com>,
	Marc Zyngier <marc.zyngier@....com>, linux-pci@...r.kernel.org,
	Liviu Dudau <Liviu.Dudau@....com>,
	linux-kernel@...r.kernel.org,
	Grant Likely <grant.likely@...aro.org>,
	Tanmay Inamdar <tinamdar@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Loc Ho <lho@....com>
Subject: Re: [PATCH v4 1/4] PCI: X-Gene: Add the APM X-Gene v1 PCIe MSI/MSIX termination driver

On Sunday 19 April 2015 11:40:09 Duc Dang wrote:
> On Fri, Apr 17, 2015 at 7:10 AM, Arnd Bergmann <arnd@...db.de> wrote:
> > On Friday 17 April 2015 02:50:07 Duc Dang wrote:
> >> +
> >> +       /*
> >> +        * MSIINTn (n is 0..F) indicates if there is a pending MSI interrupt
> >> +        * If bit x of this register is set (x is 0..7), one or more interupts
> >> +        * corresponding to MSInIRx is set.
> >> +        */
> >> +       grp_select = readl(xgene_msi->msi_regs + MSI_INT0 + (msi_grp << 16));
> >> +       while (grp_select) {
> >> +               msir_index = ffs(grp_select) - 1;
> >> +               /*
> >> +                * Calculate MSInIRx address to read to check for interrupts
> >> +                * (refer to termination address and data assignment
> >> +                * described in xgene_compose_msi_msg function)
> >> +                */
> >> +               msir_reg = (msi_grp << 19) + (msir_index << 16);
> >> +               msir_val = readl(xgene_msi->msi_regs + MSI_IR0 + msir_reg);
> >> +               while (msir_val) {
> >> +                       intr_index = ffs(msir_val) - 1;
> >> +                       /*
> >> +                        * Calculate MSI vector number (refer to the termination
> >> +                        * address and data assignment described in
> >> +                        * xgene_compose_msi_msg function)
> >> +                        */
> >> +                       hw_irq = (((msir_index * IRQS_PER_IDX) + intr_index) *
> >> +                                NR_HW_IRQS) + msi_grp;
> >> +                       virq = irq_find_mapping(xgene_msi->domain, hw_irq);
> >> +                       if (virq != 0)
> >> +                               generic_handle_irq(virq);
> >> +                       msir_val &= ~(1 << intr_index);
> >> +                       processed++;
> >> +               }
> >> +               grp_select &= ~(1 << msir_index);
> >> +       }
> >>
> >
> > As the MSI is forwarded to the GIC here, how do you maintain ordering
> > between DMA data getting forwarded from the PCI host bridge to RAM
> > with regard to the MSI handler getting entered from this code?
> 
> When device perform a DMA transfer, the order of PCIE inbound requests
> will be like this:
> 1. DMA data get transferred via PCIe inbound request
> 2. After devices issue DMA transfer request, the device fires an MSI
> interrupt by issuing another inbound write to write MSI data to MSI
> termination address.
> 
> As these 2 requests are transferred via PCIe bus in order, the DMA
> data will be all in DDR before the MSI data hit the termination
> address to trigger the MSI handler in interrupt handler code.

Obviously they appear on the PCI host bridge in order, because that
is a how PCI works. My question was about what happens then. On a lot
of SoCs, there is something like an AXI bus that uses posted
transactions between PCI and RAM, so you have a do a full manual
syncronization of ongoing PIC DMAs when the MSI catcher signals the
top-level interrupt. Do you have a bus between PCI and RAM that does
not require this, or does the MSI catcher have logic to flush all DMAs?

	Arnd
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