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Date: Mon, 8 Jan 2024 15:19:01 +0100
From: Lukas Wunner <lukas@wunner.de>
To: Vidya Sagar <vidyas@nvidia.com>
Cc: bhelgaas@google.com, alex.williamson@redhat.com, treding@nvidia.com,
	jonathanh@nvidia.com, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org, vsethi@nvidia.com, kthota@nvidia.com,
	mmaddireddy@nvidia.com, sagar.tv@gmail.com
Subject: Re: [PATCH V3] PCI: pciehp: Disable ACS Source Validation during
 hot-remove
Message-ID: <20240108141901.GA17779@wunner.de>
References: <20230111190533.29979-1-vidyas@nvidia.com>
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 <7880aa6e-9fc5-c026-138f-42bef3c48b69@nvidia.com>
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On Thu, Jan 04, 2024 at 08:01:06PM +0530, Vidya Sagar wrote:
> On 8/1/2023 1:29 AM, Lukas Wunner wrote:
> > As an alternative to disabling ACS, have you explored masking ACS
> > Violations (PCI_ERR_UNC_ACSV) upon de-enumeration of a device and
> > unmasking them after assignment of a bus number?
> 
> I explored this option and it seemed to work as expected. But, the issue
> is that this works only if the AER registers are owned by the OS. If the
> AER registers are owned by the firmware (i.e. Firmware-First approach of
> handling the errors), OS is not supposed to access the AER registers and
> there is no indication from the OS to the firmware as to when the
> enumeration is completed and time is apt to unmask the ACSViolation
> errors in the AER's Uncorrectable Error Mask register.
> Any thoughts on accommodating the Firmware-First approach also?

Are you actually using firmware-controlled AER or is it a theoretical
question?

PCI Firmware Spec r3.3 sec 4.6.12 talks about a _DSM to disable DPC
on surprise-hotplug-capable ports.  Maybe that would be an option?

BTW what happens if the system resumes from sleep and a device in
a hotplug-capable port doesn't have a bus number configured yet
(because it's been powered off and is now in D0uninitialized state)?
Could the ACS Violations then occur as well?  Do we have to mask
ACS Violations *generally* on Root Ports and Downstream Ports when
going to system sleep and unmask them after setting a bus number
in the attached device on resume?  And I suppose that would not
only be necessary for hotplug ports?

Thanks,

Lukas