Re: [PATCH] PCI: Fix 32-bit config write in Intel RP MPC ACS quirk

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Alex, author of d99321b63b1f]

On Thu, Jul 23, 2026 at 12:13:00PM +0530, Mohamad Raizudeen wrote:
> The pci_quirk_enable_intel_rp_mpc_acs() reads a 32 bit DWORD from the
> MPC register, sets bit 26 (INTEL_MPC_REG_IRBNCE), but it writes it back
> using pci_write_config_word().
> 
> Because bit 26 resides in the upper 16 bits of the 32 bit register, a 16
> bit write to offset 0xd8 completely drops the newly set bit. The quirk
> logs that it is enabling IRBNCE, but the hardware never actually
> receives the command.
> 
> Fix this by using pci_write_config_dword() to ensure the full 32 bit
> value is written back to the hardware.

Needs a Fixes: tag, probably for d99321b63b1f ("PCI: Enable quirks for PCIe
ACS on Intel PCH root ports")

That's 12 years old, surprising this hasn't been noticed before :)
Google does find several mentions of "Enabling MPC IRBNCE", so maybe there
have been issues but not debugged.

> Signed-off-by: Mohamad Raizudeen <raizudeen.kerneldev@gmail.com>
> ---
>  drivers/pci/quirks.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
> index b09f27f7846f..0d0cf6f86173 100644
> --- a/drivers/pci/quirks.c
> +++ b/drivers/pci/quirks.c
> @@ -5361,7 +5361,7 @@ static void pci_quirk_enable_intel_rp_mpc_acs(struct pci_dev *dev)
>  	if (!(mpc & INTEL_MPC_REG_IRBNCE)) {
>  		pci_info(dev, "Enabling MPC IRBNCE\n");
>  		mpc |= INTEL_MPC_REG_IRBNCE;
> -		pci_write_config_word(dev, INTEL_MPC_REG, mpc);
> +		pci_write_config_dword(dev, INTEL_MPC_REG, mpc);
>  	}
>  }
>  
> -- 
> 2.53.0
>