linux-kernel - Re: [PATCH V7 02/15] PCI: Disable MSI for Tegra194 root port

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Message-ID: <4a9dadc5-7e16-4893-c7c2-acd7f515e039@nvidia.com>
Date:   Wed, 22 May 2019 13:37:58 +0530
From:   Vidya Sagar <vidyas@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
CC:     Thierry Reding <thierry.reding@...il.com>,
        <lorenzo.pieralisi@....com>, <robh+dt@...nel.org>,
        <mark.rutland@....com>, <jonathanh@...dia.com>, <kishon@...com>,
        <catalin.marinas@....com>, <will.deacon@....com>,
        <jingoohan1@...il.com>, <gustavo.pimentel@...opsys.com>,
        <mperttunen@...dia.com>, <linux-pci@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <kthota@...dia.com>,
        <mmaddireddy@...dia.com>, <sagar.tv@...il.com>
Subject: Re: [PATCH V7 02/15] PCI: Disable MSI for Tegra194 root port

On 5/22/2019 1:06 AM, Bjorn Helgaas wrote:
> On Tue, May 21, 2019 at 10:17:26PM +0530, Vidya Sagar wrote:
>> On 5/21/2019 3:57 PM, Thierry Reding wrote:
>>> On Fri, May 17, 2019 at 06:08:33PM +0530, Vidya Sagar wrote:
>>>> Tegra194 rootports don't generate MSI interrupts for PME events and hence
>>>> MSI needs to be disabled for them to avoid root ports service drivers
>>>> registering their respective ISRs with MSI interrupt.
> 
> The service drivers (AER, hotplug, etc) don't know whether the
> interrupt is INTx or MSI; they just register their ISRs with
> pcie_device.irq.
> 
> The point of this patch is that the PCI core doesn't support devices
> that use MSI and INTx at the same time, and since this device can't
> generate MSI for PME, we have to use INTx for *all* its interrupts.
> 
>>>> Signed-off-by: Vidya Sagar <vidyas@...dia.com>
>>>> ---
>>>> Changes since [v6]:
>>>> * This is a new patch
>>>>
>>>>    drivers/pci/quirks.c | 14 ++++++++++++++
>>>>    1 file changed, 14 insertions(+)
>>>>
>>>> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
>>>> index 0f16acc323c6..28f9a0380df5 100644
>>>> --- a/drivers/pci/quirks.c
>>>> +++ b/drivers/pci/quirks.c
>>>> @@ -2592,6 +2592,20 @@ DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_NVIDIA,
>>>>    			PCI_DEVICE_ID_NVIDIA_NVENET_15,
>>>>    			nvenet_msi_disable);
>>>> +/*
>>>> + * Tegra194's PCIe root ports don't generate MSI interrupts for PME events
>>>> + * instead legacy interrupts are generated. Hence, to avoid service drivers
>>>> + * registering their respective ISRs for MSIs, need to disable MSI interrupts
>>>> + * for root ports.
>>>> + */
>>>> +static void disable_tegra194_rp_msi(struct pci_dev *dev)
>>>> +{
>>>> +	dev->no_msi = 1;
>>>> +}
>>>> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_NVIDIA, 0x1ad0, disable_tegra194_rp_msi);
>>>> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_NVIDIA, 0x1ad1, disable_tegra194_rp_msi);
>>>> +DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_NVIDIA, 0x1ad2, disable_tegra194_rp_msi);
>>>> +
>>>
>>> Later functions in this file seem to use a more consistent naming
>>> pattern, according to which the name for this would become:
>>>
>>> 	pci_quirk_nvidia_tegra194_disable_rp_msi
>>>
>>> Might be worth considering making this consistent.
>>>
>>> This could also be moved to the DWC driver to restrict this to where it
>>> is needed. In either case, this seems like a good solution, so:
>>>
>>> Reviewed-by: Thierry Reding <treding@...dia.com>
>>>
>> Ok. I'll move it to DWC driver along with name change for the quirk API.
> 
> Is there any possibility this hardware would be used in an ACPI
> system?  If so, the quirk should probably stay in drivers/pci/quirks.c
> because the DWC driver would not be present.
Yes. There is a plan to boot kernel through UEFI (using ACPI) on this system.
So, I'll move it to drivers/pci/quirks.c file.

> 
> Either way, please also add some PCIe spec references about this in
> the changelog and a comment in the code about working around this
> issue.  I think the MSI/MSI-X sections that prohibit a device from
> using both INTx and MSI/MSI-X are probably the most pertinent.
Ok. I'll take care of it in the next patch series.

> 
> The reason I want a comment about this is to discourage future
> hardware from following this example because every device that *does*
> follow this example requires a kernel update that would be otherwise
> unnecessary.
Ok. I'll take it up with our hardware engineers to have only MSI/MSI-X interrupts
getting generated for all root port received events.

> 
> Bjorn
>