Re: [PATCH v9 08/13] PCI: Save and restore the ACS Control register

[Pranjal Shrivastava <praan@google.com>]

On Fri, Sep 18, 2026 at 08:06:34PM +0000, David Matlack wrote:
> Save the ACS Control register in pci_save_state() and write it back
> in pci_restore_state(), instead of recomputing the ACS controls from
> scratch with pci_enable_acs().
> 
> This makes ACS symmetric with the rest of a device's saved state. Today
> pci_save_state() ignores ACS entirely and pci_restore_state() re-enables
> the ACS controls from the kernel's current ACS policy. As a result, a
> device can come out of a reset with different ACS controls than it went
> in with, e.g. any controls programmed outside of pci_enable_acs() are
> silently dropped.
> 
> pci_enable_acs() runs when a driver binds to a device
> (pci_dma_configure()), i.e. after pci_bus_add_device() has already saved
> the device's state. Refresh the saved ACS Control register there as
> well, otherwise a subsequent reset would revert ACS back to the
> configuration left behind by firmware.
> 
> Devices that rely on device-specific quirks to enable an ACS equivalent
> keep that configuration outside of the ACS Control register, so keep
> configuring ACS from scratch for them. Do the same for devices that have
> no saved ACS state at all.
> 
> Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: David Matlack <dmatlack@google.com>

Reviewed-by: Pranjal Shrivastava <praan@google.com>

Thanks,
Praan