Re: [Xen-devel] [PATCH for-4.5 7/8] xen/irq: Handle multiple action per IRQ

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 18 Mar 2014, Julien Grall wrote:
> Hi Stefano,
> 
> On 03/18/2014 02:06 PM, Stefano Stabellini wrote:
> >> On 03/18/2014 09:33 AM, Ian Campbell wrote:
> >>> On Mon, 2014-03-17 at 21:05 +0000, Julien Grall wrote:
> >>>> For instance for the SMMU on midway, the device tree bindings is:
> >>>>
> >>>>                smmu_sata: smmu@9,20180000 {
> >>>>                        compatible = "arm,mmu-400";
> >>>>                        reg = <0x9 0x20180000 0x10000>;
> >>>>                        mmu-masters = <&sata 0 1 2 3 4 5 6 7 8 9>;
> >>>>                        #global-interrupts = <1>;
> >>>>                        interrupts = <0 114 4 0 114 4>;
> >>>>                        calxeda,smmu-secure-config-access;
> >>>>                        arm,smmu-isolate-devices;
> >>>>               };
> >>>>
> >>>> As you can see the same interrupts is used twice:
> >>>
> >>> Is that actually valid in device tree? Or is this a quirk of the midway
> >>> DT?
> >>
> >> Yes it's valid. The interrupts property for the SMMU is described as:
> >>
> >> "Interrupt list, with the first #global-irqs entries corresponding to
> >> the global interrupts and any following entries corresponding to context
> >> interrupts, specified in order of their indexing by the SMMU.
> >>
> >> For SMMUv2 implementations, there must be exactly one interrupt per
> >> context bank. In the case of a single, combined interrupt, it must be
> >> listed multiple times."
> >>
> >> On midway there is only one IRQ with is used for both context interrupt
> >> and global interrupt. As it's the only platform on Linux with SMMU
> >> support in the device tree, we don't know if every platform will have
> >> the same behavior.
> > 
> > I understand that the SMMU might reuse the same IRQ for multiple
> > purposes. I would still handle the scenario entirely within the SMMU
> > driver. Can't we register a single handler for each of the IRQ listed
> > under the SMMU node and then figure out what was the notification for in
> > the handler? 
> > 
> 
> We will have to check in the SMMU drivers, if the IRQ was already
> registered or not (because we don't know in advance if the IRQ is
> re-used). If not, Xen will register it with a new handler.
> 
> The code to register the IRQ handler will looks like:
> 
> int num_irqs = dt_number_of_irq(smmu->node);
> dt_irqs irqs[*];
> dt_irq *irq;
> 
> for ( i = 0; i < num_irqs; i++ )
> {
>    irq = dt_device_get_irq(smmu->node, i);
>    foreach ( a in irqs )
>        if ( irq == a )
>          continue;
>    request_dt_irq();
>    irqs <= irq;
> }
> 
> I understand the point that people can badly use multiple action in the
> future, but is it a good reason to make the code more difficult to
> understand?

Can't we simply register all the irq handlers at boot time, regardless
of whether they are currently used?

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