[Intel-gfx] [PATCH] drm/i915/guc: Reserve the upper end of the Global GTT for the GuC

[Michal Wajdeczko]

On Wed, Dec 21, 2016 at 04:03:30PM +0000, Chris Wilson wrote:
> On Wed, Dec 21, 2016 at 04:30:14PM +0100, Michal Wajdeczko wrote:
> > On Wed, Dec 21, 2016 at 02:11:26PM +0000, Chris Wilson wrote:
> > > The GuC would like to own the upper portion of the GTT for itself, so
> > > exclude it from our drm_mm to prevent us using it.
> > > 
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > Cc: Michal Wajdeczko <michal.wajdeczko at intel.com>
> > > Cc: Arkadiusz Hiler <arkadiusz.hiler at intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/i915_gem_gtt.c   | 5 +++++
> > >  drivers/gpu/drm/i915/intel_guc_fwif.h | 3 +++
> > >  2 files changed, 8 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> > > index 6af9311f72f5..96bc0e83286a 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> > > @@ -3176,6 +3176,11 @@ int i915_ggtt_probe_hw(struct drm_i915_private *dev_priv)
> > >  	if (ret)
> > >  		return ret;
> > >  
> > > +	if (HAS_GUC_SCHED(dev_priv)) {
> > 
> > Btw, shouldn't we also modify HAS_GUC_SCHED macro to look at i915.enable_guc_submission
> > instead of .has_guc ? Note that Guc may be present but disabled...
> 
> And we don't really know at this point, since i915.enable_guc_submission
> is resolved later.
> 
> if (HAS_GUC_SCHED(dev_priv) && i915.enable_guc_submission) is
> conservative enough.
>  
> > > +		ggtt->base.total -= GUC_GGTT_RESERVED_TOP;
> > 
> > Hmm, while unlikely, what if RESERVED_TOP is larger than detected base.total ?
> 
> Then we are screwed :)
> 
> -       if (HAS_GUC_SCHED(dev_priv)) {
> +       if (HAS_GUC_SCHED(dev_priv) && i915.enable_guc_submission) {

Assuming that in the future HAS_GUC_SCHED will be corrected to take use sanitized
value of .enable_guc_submission, to avoid any mislead, I would rather go with

- if (HAS_GUC_SCHED(dev_priv) && i915.enable_guc_submission) {
+ if (HAS_GUC(dev_priv) && i915.enable_guc_submission) {


> +               if (WARN(ggtt->base.total < GUC_GGTT_RESERVED_TOP,
> +                        "Only found %lldMiB, but need %dMiB for the GuC",

Please update message with "GGTT" or similar to capture context what is missing ;)

> +                        ggtt->base.total >> 20, GUC_GGTT_RESERVED_TOP >> 20))
> +                       return -ENODEV;
> +
>                 ggtt->base.total -= GUC_GGTT_RESERVED_TOP;
>                 ggtt->mappable_end = min(ggtt->mappable_end, ggtt->base.total);
>         }
>  
> > > +		ggtt->mappable_end = min(ggtt->mappable_end, ggtt->base.total);
> > > +	}
> > > +
> > >  	if ((ggtt->base.total - 1) >> 32) {
> > >  		DRM_ERROR("We never expected a Global GTT with more than 32bits"
> > >  			  " of address space! Found %lldM!\n",
> > > diff --git a/drivers/gpu/drm/i915/intel_guc_fwif.h b/drivers/gpu/drm/i915/intel_guc_fwif.h
> > > index 3202b32b5638..3361d38ed859 100644
> > > --- a/drivers/gpu/drm/i915/intel_guc_fwif.h
> > > +++ b/drivers/gpu/drm/i915/intel_guc_fwif.h
> > > @@ -23,6 +23,9 @@
> > >  #ifndef _INTEL_GUC_FWIF_H
> > >  #define _INTEL_GUC_FWIF_H
> > >  
> > > +/* A small region at the top of the global GTT is reserved for use by the GuC */
> > > +#define GUC_GGTT_RESERVED_TOP		0x1200000
> > 
> > Is this region size fixed (part of Guc HW assumptions), or it can change per different FW?
> > If former, then maybe this macro should be moved to i915_guc_reg.h?
> > If latter, then maybe it is worth to explictly state that in the comment?
> 
> This is purely guess work. Feel free to supply the mising details...

If only I knew ;) maybe another day.

With the proposed changes,

Reviewed-by: Michal Wajdeczko <michal.wajdeczko at intel.com>

Thanks,
Michal