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From: Vidya Sagar <vidyas@nvidia.com>
To: Bjorn Helgaas <helgaas@kernel.org>, "Rafael J. Wysocki"
	<rafael@kernel.org>, =?iso-8859-2?Q?Krzysztof_Wilczy=F1ski?= <kw@linux.com>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>
CC: "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Manikanta
 Maddireddy <mmaddireddy@nvidia.com>, Shanker Donthineni
	<sdonthineni@nvidia.com>, Krishna Thota <kthota@nvidia.com>
Subject: [Query] ACS enablement in the DT based boot flow
Thread-Topic: [Query] ACS enablement in the DT based boot flow
Thread-Index: AQHahA4m9EX0XlmRMUW/x7Mi6fvvrw==
Date: Mon, 1 Apr 2024 10:40:15 +0000
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Hi folks,=0A=
ACS (Access Control Services) is configured for a PCI device through pci_en=
able_acs().=0A=
The first thing pci_enable_acs() checks for is whether the global flag 'pci=
_acs_enable' is set or not.=0A=
The global flag 'pci_acs_enable' is set by the function pci_request_acs().=
=0A=
=0A=
pci_enable_acs() function is called whenever a new PCI device is added to t=
he system=0A=
=0A=
 pci_enable_acs+0x4c/0x2a4=0A=
 pci_acs_init+0x38/0x60=0A=
 pci_device_add+0x1a0/0x670=0A=
 pci_scan_single_device+0xc4/0x100=0A=
 pci_scan_slot+0x6c/0x1e0=0A=
 pci_scan_child_bus_extend+0x48/0x2e0=0A=
 pci_scan_root_bus_bridge+0x64/0xf0=0A=
 pci_host_probe+0x18/0xd0=0A=
=0A=
In the case of a system that boots using device-tree blob, pci_request_acs(=
) is called when the=0A=
device driver binds with the respective device=0A=
=0A=
of_iommu_configure+0xf4/0x230=0A=
of_dma_configure_id+0x110/0x340=0A=
pci_dma_configure+0x54/0x120=0A=
really_probe+0x80/0x3e0=0A=
__driver_probe_device+0x88/0x1c0=0A=
driver_probe_device+0x3c/0x140=0A=
__device_attach_driver+0xe8/0x1e0=0A=
bus_for_each_drv+0x78/0xf0=0A=
__device_attach+0x104/0x1e0=0A=
device_attach+0x14/0x30=0A=
pci_bus_add_device+0x50/0xd0=0A=
pci_bus_add_devices+0x38/0x90=0A=
pci_host_probe+0x40/0xd0=0A=
=0A=
Since the device addition always happens first followed by the driver bindi=
ng, this flow effectively=0A=
makes sure that ACS never gets enabled.=0A=
=0A=
Ideally, I would expect the pci_request_acs() get called (probably by the O=
F framework itself) before=0A=
calling pci_enable_acs().=0A=
=0A=
This happens in the ACPI flow where pci_request_acs() is called during IORT=
 node=0A=
initialization (i.e. iort_init_platform_devices() function).=0A=
=0A=
Is this understanding correct? If yes, would it make sense to call pci_requ=
est_acs() during=0A=
OF initialization (similar to IORT initialization in ACPI flow)?=0A=
=0A=
Thanks,=0A=
Vidya Sagar=