linux-kernel - Re: [RFC PATCH] PCI/APCI: Move acpi_pci_osc

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Date:   Wed, 28 Apr 2021 20:12:07 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, rjw@...ysocki.net,
        Len Brown <lenb@...nel.org>, linux-pci@...r.kernel.org,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [RFC PATCH] PCI/APCI: Move acpi_pci_osc_support() check to
 negotiation phase

On Wed, Apr 28, 2021 at 10:21:12AM -0700, Kuppuswamy, Sathyanarayanan wrote:
> 
> 
> On 4/28/21 1:18 AM, Joerg Roedel wrote:
> > From: Joerg Roedel <jroedel@...e.de>
> > 
> > The acpi_pci_osc_support() does an _OSC query with _OSC supported set
> > to what the OS supports but a zero _OSC control value. This is
> > problematic on some platforms where the firmware allows to configure
> > whether DPC is under OS or Firmware control.
> 
> Do we run acpi_pci_osc_support() only to check whether _OSC is
> supported ? Or does it serve any other purpose.

I am not 100% sure, but to me it looks like the pure purpose of the
acpi_pci_osc_support() call was indeed to check whether the firmware is
willing to grant the OS control over some PCIe features.

> > When DPC is configured to be under OS control these platforms will
> > issue a warning in the firmware log that the OS does not support DPC.
> 
> Also, is there any other benefit from this patch other than fixing
> a warning message in firmware?

Not much other benefit, besides some removed code. But those messages
can confuse the system owner and are worth getting rid of imho.

Regards,

	Joerg