linux-kernel - Re: [PATCH] vgaarb: Use ACPI HID name to find integrated GPU

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Date:   Fri, 17 Sep 2021 11:55:00 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        mripard@...nel.org, Thomas Zimmermann <tzimmermann@...e.de>,
        "Deucher, Alexander" <alexander.deucher@....com>,
        "open list:DRM DRIVERS" <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Huacai Chen <chenhuacai@...nel.org>
Subject: Re: [PATCH] vgaarb: Use ACPI HID name to find integrated GPU

On Fri, Sep 17, 2021 at 11:49:45AM +0800, Kai-Heng Feng wrote:
> On Fri, Sep 17, 2021 at 12:38 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
> >
> > [+cc Huacai, linux-pci]
> >
> > On Wed, May 19, 2021 at 09:57:23PM +0800, Kai-Heng Feng wrote:
> > > Commit 3d42f1ddc47a ("vgaarb: Keep adding VGA device in queue") assumes
> > > the first device is an integrated GPU. However, on AMD platforms an
> > > integrated GPU can have higher PCI device number than a discrete GPU.
> > >
> > > Integrated GPU on ACPI platform generally has _DOD and _DOS method, so
> > > use that as predicate to find integrated GPU. If the new strategy
> > > doesn't work, fallback to use the first device as boot VGA.
> > >
> > > Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> > > ---
> > >  drivers/gpu/vga/vgaarb.c | 31 ++++++++++++++++++++++++++-----
> > >  1 file changed, 26 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/gpu/vga/vgaarb.c b/drivers/gpu/vga/vgaarb.c
> > > index 5180c5687ee5..949fde433ea2 100644
> > > --- a/drivers/gpu/vga/vgaarb.c
> > > +++ b/drivers/gpu/vga/vgaarb.c
> > > @@ -50,6 +50,7 @@
> > >  #include <linux/screen_info.h>
> > >  #include <linux/vt.h>
> > >  #include <linux/console.h>
> > > +#include <linux/acpi.h>
> > >
> > >  #include <linux/uaccess.h>
> > >
> > > @@ -1450,9 +1451,23 @@ static struct miscdevice vga_arb_device = {
> > >       MISC_DYNAMIC_MINOR, "vga_arbiter", &vga_arb_device_fops
> > >  };
> > >
> > > +#if defined(CONFIG_ACPI)
> > > +static bool vga_arb_integrated_gpu(struct device *dev)
> > > +{
> > > +     struct acpi_device *adev = ACPI_COMPANION(dev);
> > > +
> > > +     return adev && !strcmp(acpi_device_hid(adev), ACPI_VIDEO_HID);
> > > +}
> > > +#else
> > > +static bool vga_arb_integrated_gpu(struct device *dev)
> > > +{
> > > +     return false;
> > > +}
> > > +#endif
> > > +
> > >  static void __init vga_arb_select_default_device(void)
> > >  {
> > > -     struct pci_dev *pdev;
> > > +     struct pci_dev *pdev, *found = NULL;
> > >       struct vga_device *vgadev;
> > >
> > >  #if defined(CONFIG_X86) || defined(CONFIG_IA64)
> > > @@ -1505,20 +1520,26 @@ static void __init vga_arb_select_default_device(void)
> > >  #endif
> > >
> > >       if (!vga_default_device()) {
> > > -             list_for_each_entry(vgadev, &vga_list, list) {
> > > +             list_for_each_entry_reverse(vgadev, &vga_list, list) {
> >
> > Hi Kai-Heng, do you remember why you changed the order of this list
> > traversal?
> 
> The descending order is to keep the original behavior.
> 
> Before this patch, it breaks out of the loop as early as possible, so
> the lower numbered device is picked.
> This patch makes it only break out of the loop when ACPI_VIDEO_HID
> device is found.
> So if there are more than one device that meet "cmd & (PCI_COMMAND_IO
> | PCI_COMMAND_MEMORY)", higher numbered device will be selected.
> So the traverse order reversal is to keep the original behavior.

Can you give an example of what you mean?  I don't quite follow how it
keeps the original behavior.

If we have this:

  0  PCI_COMMAND_MEMORY set   ACPI_VIDEO_HID
  1  PCI_COMMAND_MEMORY set   ACPI_VIDEO_HID

Previously we didn't look for ACPI_VIDEO_HID, so we chose 0, now we
choose 1, which seems wrong.  In the absence of other information, I
would prefer the lower-numbered device.

Or this:

  0  PCI_COMMAND_MEMORY set
  1  PCI_COMMAND_MEMORY set   ACPI_VIDEO_HID

Previously we chose 0; now we choose 1, which does seem right, but
we'd choose 1 regardless of the order.

Or this:

  0  PCI_COMMAND_MEMORY set   ACPI_VIDEO_HID
  1  PCI_COMMAND_MEMORY set

Previously we chose 0, now we still choose 0, which seems right but
again doesn't depend on the order.

The first case, where both devices are ACPI_VIDEO_HID, is the only one
where the order matters, and I suggest that we should be using the
original order, not the reversed order.

> > I guess the list_add_tail() in vga_arbiter_add_pci_device() means
> > vga_list is generally ordered with small device numbers first and
> > large ones last.
> >
> > So you pick the integrated GPU with the largest device number.  Are
> > there systems with more than one integrated GPU?  If so, I would
> > naively expect that in the absence of an indication otherwise, we'd
> > want the one with the *smallest* device number.
> 
> There's only one integrated GPU on the affected system.
> 
> The approach is to keep the list traversal in one pass.
> Is there any regression introduce by this patch?
> If that's the case, we can separate the logic and find the
> ACPI_VIDEO_HID in second pass.

No regression, I'm just looking at Huacai's VGA patches, which affect
this area.