Re: [Freedreno] [PATCH] drm/msm: Check for the GPU IOMMU during bind

[Jordan Crouse]

On Thu, Jul 06, 2023 at 09:55:13PM +0300, Dmitry Baryshkov wrote:
> 
> On 10/03/2023 00:20, Jordan Crouse wrote:
> > While booting with amd,imageon on a headless target the GPU probe was
> > failing with -ENOSPC in get_pages() from msm_gem.c.
> > 
> > Investigation showed that the driver was using the default 16MB VRAM
> > carveout because msm_use_mmu() was returning false since headless devices
> > use a dummy parent device. Avoid this by extending the existing is_a2xx
> > priv member to check the GPU IOMMU state on all platforms and use that
> > check in msm_use_mmu().
> > 
> > This works for memory allocations but it doesn't prevent the VRAM carveout
> > from being created because that happens before we have a chance to check
> > the GPU IOMMU state in adreno_bind.
> > 
> > There are a number of possible options to resolve this but none of them are
> > very clean. The easiest way is to likely specify vram=0 as module parameter
> > on headless devices so that the memory doesn't get wasted.
> 
> This patch was on my plate for quite a while, please excuse me for
> taking it so long.

No worries. I'm also chasing a bunch of other stuff too.

> I see the following problem with the current code. We have two different
> instances than can access memory: MDP/DPU and GPU. And each of them can
> either have or miss the MMU.
> 
> For some time I toyed with the idea of determining whether the allocated
> BO is going to be used by display or by GPU, but then I abandoned it. We
> can have display BOs being filled by GPU, so handling it this way would
> complicate things a lot.
> 
> This actually rings a tiny bell in my head with the idea of splitting
> the display and GPU parts to two different drivers, but I'm not sure
> what would be the overall impact.

As I now exclusively work on headless devices I would be 100% for this,
but I'm sure that our laptop friends might not agree :)

> More on the msm_use_mmu() below.
> 
> > 
> > Signed-off-by: Jordan Crouse <jorcrous@xxxxxxxxxx>
> > ---
> > 
> >   drivers/gpu/drm/msm/adreno/adreno_device.c | 6 +++++-
> >   drivers/gpu/drm/msm/msm_drv.c              | 7 +++----
> >   drivers/gpu/drm/msm/msm_drv.h              | 2 +-
> >   3 files changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/msm/adreno/adreno_device.c b/drivers/gpu/drm/msm/adreno/adreno_device.c
> > index 36f062c7582f..4f19da28f80f 100644
> > --- a/drivers/gpu/drm/msm/adreno/adreno_device.c
> > +++ b/drivers/gpu/drm/msm/adreno/adreno_device.c
> > @@ -539,7 +539,11 @@ static int adreno_bind(struct device *dev, struct device *master, void *data)
> >       DBG("Found GPU: %u.%u.%u.%u", config.rev.core, config.rev.major,
> >               config.rev.minor, config.rev.patchid);
> > 
> > -     priv->is_a2xx = config.rev.core == 2;
> > +     /*
> > +      * A2xx has a built in IOMMU and all other IOMMU enabled targets will
> > +      * have an ARM IOMMU attached
> > +      */
> > +     priv->has_gpu_iommu = config.rev.core == 2 || device_iommu_mapped(dev);
> >       priv->has_cached_coherent = config.rev.core >= 6;
> > 
> >       gpu = info->init(drm);
> > diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
> > index aca48c868c14..a125a351ec90 100644
> > --- a/drivers/gpu/drm/msm/msm_drv.c
> > +++ b/drivers/gpu/drm/msm/msm_drv.c
> > @@ -318,11 +318,10 @@ bool msm_use_mmu(struct drm_device *dev)
> >       struct msm_drm_private *priv = dev->dev_private;
> > 
> >       /*
> > -      * a2xx comes with its own MMU
> > -      * On other platforms IOMMU can be declared specified either for the
> > -      * MDP/DPU device or for its parent, MDSS device.
> > +      * Return true if the GPU or the MDP/DPU or parent MDSS device has an
> > +      * IOMMU
> >        */
> > -     return priv->is_a2xx ||
> > +     return priv->has_gpu_iommu ||
> >               device_iommu_mapped(dev->dev) ||
> >               device_iommu_mapped(dev->dev->parent);
> 
> I have a generic feeling that both old an new code is not fully correct.
> Please correct me if I'm wrong:
> 
> We should be using VRAM, if either of the blocks can not use remapped
> memory. So this should have been:
> 
> bool msm_use_mmu()
> {
>  if (!gpu_has_iommu)
>    return false;
> 
>  if (have_display_part && !display_has_mmu())
>    return false;
> 
>  return true;
> }
> 
> What do you think.

I would have to see (and try) the real code but that seems like it might
be reasonable. I would like to hear from some of the a2xx users too
because this mostly affects them. On 3xx and newer you've always had the
option of not having a MMU for GPU or display but I can't think of any
use cases where you wouldn't want it.

> --
> With best wishes
> Dmitry
> 

Jordan