[PATCH v2 09/22] drm/tegra: Don't use IOMMU on Tegra20

[Dmitry Osipenko]

On 14.06.2017 13:22, Dmitry Osipenko wrote:
> On 14.06.2017 10:39, Erik Faye-Lund wrote:
>> On Wed, Jun 14, 2017 at 1:15 AM, Dmitry Osipenko <digetx at gmail.com> wrote:
>>> There is no IOMMU on Tegra20, instead a GART would be picked as an IOMMU
>>> provider.
>>>
>>> Signed-off-by: Dmitry Osipenko <digetx at gmail.com>
>>> Acked-by: Joerg Roedel <jroedel at suse.de>
>>> ---
>>>  drivers/gpu/drm/tegra/drm.c | 3 ++-
>>>  drivers/gpu/host1x/dev.c    | 3 ++-
>>>  2 files changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/tegra/drm.c b/drivers/gpu/drm/tegra/drm.c
>>> index e999391aedc9..aa7988dcc28f 100644
>>> --- a/drivers/gpu/drm/tegra/drm.c
>>> +++ b/drivers/gpu/drm/tegra/drm.c
>>> @@ -131,7 +131,8 @@ static int tegra_drm_load(struct drm_device *drm, unsigned long flags)
>>>         if (!tegra)
>>>                 return -ENOMEM;
>>>
>>> -       if (iommu_present(&platform_bus_type)) {
>>> +       if (iommu_present(&platform_bus_type) &&
>>> +           !of_machine_is_compatible("nvidia,tegra20")) {
>>>                 u64 carveout_start, carveout_end, gem_start, gem_end;
>>>                 struct iommu_domain_geometry *geometry;
>>>                 unsigned long order;
>>> diff --git a/drivers/gpu/host1x/dev.c b/drivers/gpu/host1x/dev.c
>>> index f05ebb14fa63..6a805ed908c3 100644
>>> --- a/drivers/gpu/host1x/dev.c
>>> +++ b/drivers/gpu/host1x/dev.c
>>> @@ -177,7 +177,8 @@ static int host1x_probe(struct platform_device *pdev)
>>>                 return err;
>>>         }
>>>
>>> -       if (iommu_present(&platform_bus_type)) {
>>> +       if (iommu_present(&platform_bus_type) &&
>>> +           !of_machine_is_compatible("nvidia,tegra20")) {
>>>                 struct iommu_domain_geometry *geometry;
>>>                 unsigned long order;
>>>
>>
>> This doesn't feel great... The commit message says there's no IOMMU,
>> but iommu_present says otherwise. I know there's some more subtleties
>> here, and the commit message does hint at this. But...
>>
>> If we don't want to treat the GART as an IOMMU, shouldn't we somehow
>> make sure iommu_present() doesn't return true in these cases (or
>> perhaps make something like tegra_use_iommu(), with a comment
>> explaining why we don't want to allow the GART to be treated like a
>> proper IOMMU)? These seems to be the only Tegra-specific calls to
>> iommu_present()...
>>
>> That being said, the patch seems to have the right effect...
>>
> 
> We don't want to treat the GART as an IOMMU right now, but I'd want to change
> that in the (near?) future, so it's a kinda trivial preparation patch that
> restores the GART driver and these of_machine_is_compatible() are supposed to go
> away later. Probably I should mention this in the commit message(?).
> 
> I think we can add a Tegra20 specific IOCTL for an allocation of GART-able
> memory and use it for a stuff like opentegra's EXA, its fallback allocations
> migration is a pita and reserved CMA is never enough =)
> 

I have hacked the DRM driver to work with GART correctly for the IOMMU
allocations, the grate tests and opentegra are working fine utilizing the GART,
CMA is barely utilized and kmsg shows GART's map/unmap debug messages that I've
enabled in the driver. So I think GART should be really useful for us.

Actually, now I'm thinking that it should be better to postpone the restoring of
the GART driver till we have a full solution, maybe next merge window. I'll drop
these two patches from the series in v3 that I'm going to send shortly.

-- 
Dmitry