linux-kernel - Re: [Query] ACS enablement in the DT based boot flow

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Date: Wed, 10 Apr 2024 13:10:59 +0530
From: Vidya Sagar <vidyas@...dia.com>
To: Bjorn Helgaas <helgaas@...nel.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, Krzysztof Wilczyński <kw@...ux.com>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>
Cc: "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Manikanta Maddireddy <mmaddireddy@...dia.com>,
 Shanker Donthineni <sdonthineni@...dia.com>,
 Krishna Thota <kthota@...dia.com>
Subject: Re: [Query] ACS enablement in the DT based boot flow

Hi folks,
any thoughts on this?

On 01-04-2024 16:10, Vidya Sagar wrote:
> Hi folks,
> ACS (Access Control Services) is configured for a PCI device through pci_enable_acs().
> The first thing pci_enable_acs() checks for is whether the global flag 'pci_acs_enable' is set or not.
> The global flag 'pci_acs_enable' is set by the function pci_request_acs().
>
> pci_enable_acs() function is called whenever a new PCI device is added to the system
>
>   pci_enable_acs+0x4c/0x2a4
>   pci_acs_init+0x38/0x60
>   pci_device_add+0x1a0/0x670
>   pci_scan_single_device+0xc4/0x100
>   pci_scan_slot+0x6c/0x1e0
>   pci_scan_child_bus_extend+0x48/0x2e0
>   pci_scan_root_bus_bridge+0x64/0xf0
>   pci_host_probe+0x18/0xd0
>
> In the case of a system that boots using device-tree blob, pci_request_acs() is called when the
> device driver binds with the respective device
>
> of_iommu_configure+0xf4/0x230
> of_dma_configure_id+0x110/0x340
> pci_dma_configure+0x54/0x120
> really_probe+0x80/0x3e0
> __driver_probe_device+0x88/0x1c0
> driver_probe_device+0x3c/0x140
> __device_attach_driver+0xe8/0x1e0
> bus_for_each_drv+0x78/0xf0
> __device_attach+0x104/0x1e0
> device_attach+0x14/0x30
> pci_bus_add_device+0x50/0xd0
> pci_bus_add_devices+0x38/0x90
> pci_host_probe+0x40/0xd0
>
> Since the device addition always happens first followed by the driver binding, this flow effectively
> makes sure that ACS never gets enabled.
>
> Ideally, I would expect the pci_request_acs() get called (probably by the OF framework itself) before
> calling pci_enable_acs().
>
> This happens in the ACPI flow where pci_request_acs() is called during IORT node
> initialization (i.e. iort_init_platform_devices() function).
>
> Is this understanding correct? If yes, would it make sense to call pci_request_acs() during
> OF initialization (similar to IORT initialization in ACPI flow)?
>
> Thanks,
> Vidya Sagar