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

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Message-ID: <ZXrA_OocbBkFAkM-@macbook>
Date:   Thu, 14 Dec 2023 09:46:52 +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 Thu, Dec 14, 2023 at 07:08:32AM +0000, Chen, Jiqian wrote:
> On 2023/12/13 20:12, Roger Pau Monné wrote:
> > 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 the maintainers of sysfs can give some suggest places to initialize the value of gsi.
> 
> > 
> >>>
> >>> 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.
> Sure, I have validated it on my side. Both the "Static assignment for built-in xen-pciback(xen-pciback.hide=(SBDF))" and the "Dynamic assignment with xl(sudo modprobe xen-pciback & sudo xl pci-assignable-add SBDF)" can work fine with current implementation.

Oh, OK, if that's the case I don't have much objection in doing the
initialization in acpi_pci_irq_enable(), that's an internal Linux
detail.  I mostly care about the GSI being exposed in sysfs in a
non-Xen specific way.

Thanks, Roger.