Re: [PATCH] x86, aperture: Check for GART before accessing GART registers

From: Aravind Gopalakrishnan
Date: Tue Apr 07 2015 - 16:34:32 EST

On 4/7/2015 9:57 AM, Borislav Petkov wrote:
On Tue, Apr 07, 2015 at 09:46:26AM -0500, Aravind Gopalakrishnan wrote:
Okay. I'll do that and correct the typos Ingo pointed out earlier and
Btw, I think you should do the same in early_gart_iommu_check() too.

Doing the testing this way would mean that we first are testing for GART
hw presence and then do the rest of checks. If no GART hw, the rest of
the checks are meaningless.

Also, when testing do a "pci=noearly" boot which should make


true and thus test that path too.

Here are results from further testing:
1. on platforms with both iommu and gart
- with pci=noearly, we break out of init routines in aperture_64.c early. amd_iommu_init() will run through it's init routine.
- if amd_iommu_init() fails somewhere, we fall back to gart_iommu_init()
- gart_iommu_init() fails since gart_iommu_aperture is not set.
- fall back to swiotlb.
- with amd_iommu=off
- init routines in aperture_64.c run fine as both amd_gart_present() and early_pci_allowed() are true
- amd_iommu_detect() fails due to command line arg.
- fall back to gart iommu
- with pci=noearly and amd_iommu=off
- break out of aperture_64.c init routines, and amd_iommu_detect() fails.
- fall back to swiotlb

2. on platform with no gart but iommu present,
- pci=noearly option is not relevant as we break before that due to !amd_gart_present()
- if amd_iommu_init() fails somewhere, we fall back to swiotlb, else use iommu

3. on platforms with no gart and no iommu
- use swiotlb regardless of any command line options passed

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