[Intel-gfx] [PATCH] Fix intel_detect_pch() to work in xen environment.

[G.R.]

On Wed, Dec 19, 2012 at 12:53 AM, Jesse Barnes <jbarnes at virtuousgeek.org> wrote:
> On Wed, 19 Dec 2012 00:49:28 +0800
> "G.R." <firemeteor at users.sourceforge.net> wrote:
>
>> Hi guys,
>>
>> In XEN HVM guest, there is always an emulated PIIX3 ISA bridge on slot 01.0.
>> This shadows the PCH ISA bridge on 1f.0 with the current
>> intel_detect_pch() implementation.
>> The issue can be easily solved by looping through all the ISA bridges
>> until the first match is found, instead of just check against the
>> first one.
>>
>> Here I attach the patch I used locally. It's created on Torvalds's git.
>> Looking forward to your comments.
>>
>> Thanks,
>> Timothy
>>
>> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
>> index 530db83..3f7e5fb 100644
>> --- a/drivers/gpu/drm/i915/i915_drv.c
>> +++ b/drivers/gpu/drm/i915/i915_drv.c
>> @@ -408,9 +408,11 @@ void intel_detect_pch(struct drm_device *dev)
>>          * underneath. This is a requirement from virtualization team.
>>          */
>>         pch = pci_get_class(PCI_CLASS_BRIDGE_ISA << 8, NULL);
>> -       if (pch) {
>> +       while (pch) {
>> +               struct pci_dev * curr = pch;
>>                 if (pch->vendor == PCI_VENDOR_ID_INTEL) {
>>                         unsigned short id;
>> +                       unsigned found = 1;
>>                         id = pch->device & INTEL_PCH_DEVICE_ID_MASK;
>>                         dev_priv->pch_id = id;
>>
>> @@ -440,10 +442,20 @@ void intel_detect_pch(struct drm_device *dev)
>>                                 dev_priv->num_pch_pll = 0;
>>                                 DRM_DEBUG_KMS("Found LynxPoint LP PCH\n");
>>                                 WARN_ON(!IS_HASWELL(dev));
>> +                       } else {
>> +                               found = 0;
>> +                       }
>> +                       if (found) {
>> +                               BUG_ON(dev_priv->num_pch_pll > I915_NUM_PLLS);
>> +                               pci_dev_put(pch);
>> +                               break;
>>                         }
>> -                       BUG_ON(dev_priv->num_pch_pll > I915_NUM_PLLS);
>>                 }
>> -               pci_dev_put(pch);
>> +               pch = pci_get_class(PCI_CLASS_BRIDGE_ISA << 8, curr);
>> +               pci_dev_put(curr);
>> +       }
>> +       if (!pch) {
>> +               DRM_DEBUG_KMS("No PCH found?\n");
>>         }
>>  }
>>
>
> I'd like to see a comment about this being for Xen in here, and I
> wonder if there are other places where we might have to worry about the
> Xen implementation.  In that case, setting a flag in dev_priv when we
> don't find the PCH where we expect would be a good idea too.
>

I can add a comment here if the overall idea is acceptable to you.
But there is already a comment mentioning that the ISA bridge check is
for virtualization:

 404         /*
 405          * The reason to probe ISA bridge instead of Dev31:Fun0 is to
 406          * make graphics device passthrough work easy for VMM, that only
 407          * need to expose ISA bridge to let driver know the real hardware
 408          * underneath. This is a requirement from virtualization team.
 409          */

> Ack on the general idea though; I'd like us to be able to run under Xen
> without modification.
>

Stefano may be able to comment if it's feasible to achieve zero
modification in this case.
I believe this has something to do with getting rid of the PIIX3
device provided by qemu.

But generally I think it's very hard to achieve perfect emulation.
You can't always foresee what assumption a guest driver would make.
Maybe we need some compromise.

> Jesse
>
> --
> Jesse Barnes, Intel Open Source Technology Center