linux-kernel - Re: [RFC PATCH v2 09/10] irqchip/gic: Convert to handle_strict_flow

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Message-ID: <87lf7bb1ek.mognet@arm.com>
Date:   Tue, 15 Jun 2021 16:20:03 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>
Subject: Re: [RFC PATCH v2 09/10] irqchip/gic: Convert to handle_strict_flow_irq()

On 01/06/21 11:25, Valentin Schneider wrote:
> On 27/05/21 13:21, Marc Zyngier wrote:
>> On Tue, 25 May 2021 18:32:54 +0100,
>> Valentin Schneider <valentin.schneider@....com> wrote:
>>> @@ -1116,8 +1116,16 @@ static void gic_init_chip(struct gic_chip_data *gic, struct device *dev,
>>>
>>>      if (use_eoimode1) {
>>>              gic->chip.irq_mask = gic_eoimode1_mask_irq;
>>> +		gic->chip.irq_ack = gic_eoi_irq;
>>>              gic->chip.irq_eoi = gic_eoimode1_eoi_irq;
>>>              gic->chip.irq_set_vcpu_affinity = gic_irq_set_vcpu_affinity;
>>> +
>>> +		/*
>>> +		 * eoimode0 shouldn't expose FLOW_MASK because the priority
>>> +		 * drop is undissociable from the deactivation, and we do need
>>> +		 * the priority drop to happen within the flow handler.
>>> +		 */
>>> +		gic->chip.flags |= IRQCHIP_AUTOMASKS_FLOW | IRQCHIP_EOI_THREADED;
>>>      }
>>>
>>>      if (gic == &gic_data[0]) {
>>
>> How about GICv2M, GICv3-MBI, and the collection of widget that build a
>> domain on top of a GIC domain? I'm worried that they now all need
>> updating one way or another...
>>
>
> Hmph, that's a good point. It's been a while since I've last stared at the
> v2m, I'll go try to page that back in.
>

It's taken me a while to get back to this, apologies. Here's where I'm at:

At the very least these need the +.irq_ack() treatment, same as the ITS
chips. We can get around this by giving msi_domain_update_chip_ops() some
invoke-first-non-NULL default callbacks, as you've suggested in:

  http://lore.kernel.org/r/87y2c0s748.wl-maz@kernel.org

Now, looking at this made me think about which irq_chip flags are being
used where, and, well...

PCI-MSI IRQs are deemed 'oneshot safe', but platform-MSI ones aren't. So
for instance, if a GICv2M pMSI IRQ gets force-threaded, we'll make it
IRQS_ONESHOT. However, this is still just a glorified SPI as all mask, ack
and eoi operations will be the root chip's, so we should be able to apply
the eoimode=1 automask trickery to it. This won't happen with the current
patches, since the ->chip we'll seeing in handle_strict_flow_irq() will be
gicv2m_pmsi_irq_chip.

We *could* give that one the required flags, but what actually matters for
the automask thing are the flags of first chip in the hiearachy that has
"proper" ack+eoi callbacks. I don't see a nice way of handling this right
now...

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