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Subject: Re: [PATCH V6 02/15] PCI/PME: Export pcie_pme_disable_msi() &
 pcie_pme_no_msi() APIs
To:     Bjorn Helgaas <helgaas@kernel.org>
CC:     Christoph Hellwig <hch@infradead.org>, <lorenzo.pieralisi@arm.com>,
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From:   Vidya Sagar <vidyas@nvidia.com>
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Date:   Fri, 17 May 2019 13:49:49 +0530
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On 5/16/2019 7:04 PM, Bjorn Helgaas wrote:
> On Tue, May 14, 2019 at 09:00:19AM +0530, Vidya Sagar wrote:
>> On 5/13/2019 12:55 PM, Christoph Hellwig wrote:
>>> On Mon, May 13, 2019 at 10:36:13AM +0530, Vidya Sagar wrote:
>>>> Export pcie_pme_disable_msi() & pcie_pme_no_msi() APIs to enable drivers
>>>> using these APIs be able to build as loadable modules.
>>>
>>> But this is a global setting.  If you root port is broken you need
>>> a per-rootport quirk instead.
>>>
>> There is nothing broken in Tegra194 root port as such, rather, this
>> is more of software configuration choice and we are going with
>> legacy interrupts than MSI interrupts (as Tegra194 doesn't support
>> raising PME interrupts through MSI and please note that this doesn't
>> mean root port is broken).
> 
> I think the port *is* broken.  PCIe r4.0, sec 6.1.6, says
> 
>    If the Root Port is enabled for edge-triggered interrupt signaling
>    using MSI or MSI-X, an interrupt message must be sent every time the
>    logical AND of the following conditions transitions from FALSE to
>    TRUE:
> 
>      * The associated vector is unmasked (not applicable if MSI does
>        not support PVM).
> 
>      * The PME Interrupt Enable bit in the Root Control register is set
>        to 1b.
> 
>      * The PME Status bit in the Root Status register is set.
> 
> The Tegra194 root port advertises MSI support, so the above should
> apply.
I had a discussion with our hardware engineers and we are of the opinion
that the root port is not really broken w.r.t MSI as spec doesn't clearly
say that if root port advertises MSI support, it must generate MSI interrupts
for PME. All that it says is, if MSI is enabled, then MSI should be raised
for PME events. Here, by 'enable', we understand that as enabling at
hardware level to generate MSI interrupt which is not the case with Tegra194.
In Tegra194, root port is enabled to generate MSI only for hot-plug events and
legacy interrupts are used for PME, AER. Having said that I'm fine to add a
quick based on Vendor-ID and Device-IDs of root port in Tegra194 to set
pdev->no_msi to '1'.

> 
>> Since Tegra194 has only Synopsys DesignWare core based host
>> controllers and not any other hosts, I think it is fine to call this
>> API in driver.
> 
> It's fine to add a per-device quirk to set pdev->no_msi or something
> similar.
> 
> Bjorn
>