[PATCH 04/12] drm/i915: Bypass LMEMBAR/GTTMMADR for MTL stolen memory access

[Ville Syrjälä]

On Wed, Dec 13, 2023 at 11:09:38AM +0200, Joonas Lahtinen wrote:
> Quoting Ville Syrjala (2023-12-13 02:42:29)
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > 
> > On MTL accessing stolen memory via the BARs is somehow borked,
> > and it can hang the machine. As a workaround let's bypass the
> > BARs and just go straight to DSMBASE/GSMBASE instead.
> > 
> > Note that on every other platform this itself would hang the
> > machine, but on MTL the system firmware is expected to relax
> > the access permission guarding stolen memory to enable this
> > workaround, and thus direct CPU accesses should be fine.
> 
> Shouldn't this have a proper workaround number assigned?

I think there are various numbers, half of which I couldn't even
find in any database, and the other half I couldn't access for
whatever reason. So dunno what situation really is apart from
the firmware clearly implemening its part (since I can poke 
DSM/GSM directly without killing the machine).

RK do you know what we should call this?

> 
> Regards, Joonas
> 
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > ---
> >  drivers/gpu/drm/i915/gem/i915_gem_stolen.c | 11 ++++++++++-
> >  drivers/gpu/drm/i915/gt/intel_ggtt.c       | 13 ++++++++++++-
> >  2 files changed, 22 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_stolen.c b/drivers/gpu/drm/i915/gem/i915_gem_stolen.c
> > index ee237043c302..252fe5cd6ede 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_stolen.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_stolen.c
> > @@ -941,7 +941,16 @@ i915_gem_stolen_lmem_setup(struct drm_i915_private *i915, u16 type,
> >                 dsm_size = ALIGN_DOWN(lmem_size - dsm_base, SZ_1M);
> >         }
> >  
> > -       if (pci_resource_len(pdev, GEN12_LMEM_BAR) < lmem_size) {
> > +       if (IS_METEORLAKE(i915)) {
> > +               /*
> > +                * Workaround: access via BAR can hang MTL, go directly to DSM.
> > +                *
> > +                * Normally this would not work but on MTL the system firmware
> > +                * should have relaxed the access permissions sufficiently.
> > +                */
> > +               io_start = intel_uncore_read64(uncore, GEN12_DSMBASE) & GEN12_BDSM_MASK;
> > +               io_size = dsm_size;
> > +       } else if (pci_resource_len(pdev, GEN12_LMEM_BAR) < lmem_size) {
> >                 io_start = 0;
> >                 io_size = 0;
> >         } else {
> > diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > index 21a7e3191c18..ab71d74ec426 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> > @@ -24,6 +24,7 @@
> >  #include "intel_ring.h"
> >  #include "i915_drv.h"
> >  #include "i915_pci.h"
> > +#include "i915_reg.h"
> >  #include "i915_request.h"
> >  #include "i915_scatterlist.h"
> >  #include "i915_utils.h"
> > @@ -1152,13 +1153,23 @@ static unsigned int gen6_gttadr_offset(struct drm_i915_private *i915)
> >  static int ggtt_probe_common(struct i915_ggtt *ggtt, u64 size)
> >  {
> >         struct drm_i915_private *i915 = ggtt->vm.i915;
> > +       struct intel_uncore *uncore = ggtt->vm.gt->uncore;
> >         struct pci_dev *pdev = to_pci_dev(i915->drm.dev);
> >         phys_addr_t phys_addr;
> >         u32 pte_flags;
> >         int ret;
> >  
> >         GEM_WARN_ON(pci_resource_len(pdev, GEN4_GTTMMADR_BAR) != gen6_gttmmadr_size(i915));
> > -       phys_addr = pci_resource_start(pdev, GEN4_GTTMMADR_BAR) + gen6_gttadr_offset(i915);
> > +       /*
> > +        * Workaround: access via BAR can hang MTL, go directly to GSM.
> > +        *
> > +        * Normally this would not work but on MTL the system firmware
> > +        * should have relaxed the access permissions sufficiently.
> > +        */
> > +       if (IS_METEORLAKE(i915))
> > +               phys_addr = intel_uncore_read64(uncore, GEN12_GSMBASE) & GEN12_BDSM_MASK;
> > +       else
> > +               phys_addr = pci_resource_start(pdev, GEN4_GTTMMADR_BAR) + gen6_gttadr_offset(i915);
> >  
> >         if (needs_wc_ggtt_mapping(i915))
> >                 ggtt->gsm = ioremap_wc(phys_addr, size);
> > -- 
> > 2.41.0
> > 

-- 
Ville Syrjälä
Intel