linux-kernel - Re: [PATCH v11 2/4] PCI: Factor out pci_bus_wait

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Message-ID: <20170821202338.GD28977@bhelgaas-glaptop.roam.corp.google.com>
Date:   Mon, 21 Aug 2017 15:23:38 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        Timur Tabi <timur@...eaurora.org>,
        linux-kernel@...r.kernel.org,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-arm-msm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v11 2/4] PCI: Factor out pci_bus_wait_crs()

On Mon, Aug 21, 2017 at 03:37:06PM -0400, Sinan Kaya wrote:
> On 8/21/2017 3:18 PM, Bjorn Helgaas wrote:
> ...
> if (pci_bus_crs_pending(id))
> 	return pci_bus_wait_crs(dev->bus, dev->devfn, &id, 60000);
> 
> > I think that makes sense.  We'd want to check for CRS SV being
> > enabled, e.g., maybe read PCI_EXP_RTCTL_CRSSVE back in
> > pci_enable_crs() and cache it somewhere.  Maybe a crs_sv_enabled bit
> > in the root port's pci_dev, and check it with something like what
> > pcie_root_rcb_set() does?
> > 
> 
> You can observe CRS under the following conditions
> 
> 1. root port <-> endpoint 
> 2. bridge <-> endpoint 
> 3. root port<->bridge
> 
> I was relying on the fact that we are reading 0x001 as an indication that
> this device detected CRS. Maybe, this is too indirect.
> 
> If we also want to capture the capability, I think the right thing is to
> check the parent capability.
> 
> bool pci_bus_crs_vis_supported(struct pci_dev *bridge)
> {
> 	if (device type(bridge) == root port)
> 		return read(root_crs_register_reg);
> 
> 	if (device type(bridge) == switch)
> 		return read(switch_crs_register);

I don't understand this part.  AFAIK, CRS SV is only a feature of root
ports.  The capability and enable bits are in the Root Capabilities
and Root Control registers.

It's certainly true that a device below a switch can respond with a
CRS completion, but the switch is not the requester, and my
understanding is that it would not take any action on the completion
other than passing it upstream.