linux-kernel - Re: [RFC 2/4] PCI, MSI: Optionally free legacy PCI IRQ when enabling MSI/MSI-X

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Date:	Wed, 20 May 2015 11:06:25 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	"Rafael J . Wysocki" <rjw@...ysocki.net>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lv Zheng <lv.zheng@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, "x86 @ kernel . org" <x86@...nel.org>
Subject: Re: [RFC 2/4] PCI, MSI: Optionally free legacy PCI IRQ when enabling
 MSI/MSI-X

On 2015/5/16 5:02, Thomas Gleixner wrote:
> On Thu, 7 May 2015, Jiang Liu wrote:
> 
>> Once PCI MSI/MSI-X is enabled by the device driver, PCI device won't
>> make use of legacy PCI IRQ until PCI MSI/MSI-X is disabled again.
>> So optionally free legacy PCI IRQ when enabling MSI/MSI-X and reallocate
>> when disabling MSI/MSI-X.
> 
> This is a bit odd. With your proposed change we'll have:
> 
>      alloc_legacy_irq()
> 
>      msi[x]_enable()
> 	free_legacy_irq()
> 
>      msi[x]_disable()
> 	alloc_legacy_irq()
Hi Thomas,
	It's for safety. I'm not sure whether the device driver will
make use of legacy IRQ after calling msi[x]_disable(). I have concerns
about following pattern in PCI device drivers:
---------------------------------------------------
if (enable_msi() == SUCCESS) {
	if (allocate_resource_for_msi() == SUCCESS)
		return;
	disable_msi();
}
use_legacy_irq()

Thanks!
Gerry

> 
> And after that we shut down the device which will free the legacy irq
> again.
> 
> Shouldn't we allocate the legacy irq only if we really need it?
> 
> Thanks,
> 
> 	tglx
> 
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