linux-kernel - Re: [RFC KERNEL PATCH v3 3/3] PCI/sysfs: Add gsi sysfs for pci

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Date:   Wed, 13 Dec 2023 13:12:09 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     "Chen, Jiqian" <Jiqian.Chen@....com>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Koenig, Christian" <Christian.Koenig@....com>,
        "Huang, Ray" <Ray.Huang@....com>
Subject: Re: [RFC KERNEL PATCH v3 3/3] PCI/sysfs: Add gsi sysfs for pci_dev

On Wed, Dec 13, 2023 at 03:31:21AM +0000, Chen, Jiqian wrote:
> On 2023/12/12 17:18, Roger Pau Monné wrote:
> > On Tue, Dec 12, 2023 at 06:34:27AM +0000, Chen, Jiqian wrote:
> >>
> >> On 2023/12/12 01:57, Roger Pau Monné wrote:
> >>> On Mon, Dec 11, 2023 at 12:15:19AM +0800, Jiqian Chen wrote:
> >>>> There is a need for some scenarios to use gsi sysfs.
> >>>> For example, when xen passthrough a device to dumU, it will
> >>>> use gsi to map pirq, but currently userspace can't get gsi
> >>>> number.
> >>>> So, add gsi sysfs for that and for other potential scenarios.
> >>>>
> >>>> Co-developed-by: Huang Rui <ray.huang@....com>
> >>>> Signed-off-by: Jiqian Chen <Jiqian.Chen@....com>
> >>>> ---
> >>>>  drivers/acpi/pci_irq.c  |  1 +
> >>>>  drivers/pci/pci-sysfs.c | 11 +++++++++++
> >>>>  include/linux/pci.h     |  2 ++
> >>>>  3 files changed, 14 insertions(+)
> >>>>
> >>>> diff --git a/drivers/acpi/pci_irq.c b/drivers/acpi/pci_irq.c
> >>>> index 630fe0a34bc6..739a58755df2 100644
> >>>> --- a/drivers/acpi/pci_irq.c
> >>>> +++ b/drivers/acpi/pci_irq.c
> >>>> @@ -449,6 +449,7 @@ int acpi_pci_irq_enable(struct pci_dev *dev)
> >>>>  		kfree(entry);
> >>>>  		return 0;
> >>>>  	}
> >>>> +	dev->gsi = gsi;
> >>>
> >>> It would be better if the gsi if fetched without requiring calling
> >>> acpi_pci_irq_enable(), as the gsi doesn't require the interrupt to be
> >>> enabled.  The gsi is known at boot time and won't change for the
> >>> lifetime of the device.
> >> Do you have any suggest places to do this?
> > 
> > I'm not an expert on this, but drivers/pci/pci-sysfs.c would seem like
> > a better place, together with the rest of the resources.
> I'm not familiar with this too. But it seems pci-sysfs.c only creates sysfs node and supports the read/write method without initializing the values.
> If want to initialize the value of gsi here. An approach to initialize it is to call acpi_pci_irq_lookup to get gsi number when the first time it is read?

Hm, maybe, I don't really have much experience with sysfs, so don't
know how nodes are usually initialized.

> > 
> > Maybe my understanding is incorrect, but given the suggested placement
> > in acpi_pci_irq_enable() I think the device would need to bind the
> > interrupt in order for the gsi node to appear on sysfs?
> No, gsi sysfs has existed there, in acpi_pci_irq_enable is to initialize the value of gsi.
> 
> > 
> > Would the current approach work if the device is assigned to pciback
> > on the kernel command line, and thus never owned by any driver in
> > dom0?
> If assigned to pciback, I think pciback will enable the device, and then acpi_pci_irq_enable will be called, and then the gsi will be initialized. So, current can work.

This needs checking to be sure, I'm certainly not that familiar.  You
would need to at least test that it works properly when the device is
hidden using xen-pciback.hide=(SBDF) in the Linux kernel command line.

Thanks, Roger.