[Intel-gfx] [PATCH v2] drm/i915/huc: Don't try to check HuC status if it's not loaded

[Chris Wilson]

Quoting Michal Wajdeczko (2019-05-20 11:24:37)
> On Mon, 20 May 2019 11:35:26 +0200, Chris Wilson  
> <chris at chris-wilson.co.uk> wrote:
> 
> > Quoting Michal Wajdeczko (2019-05-19 22:50:43)
> >> If we never attempted to load HuC firmware, or we already wedged
> >> or reset GuC/HuC, then there is no reason to wake up the device
> >> to check one bit in the register that will be for sure cleared.
> >>
> >> v2: check if HuC was enabled; subtle change in ABI
> >>     reuse hus_is_load helper
> >>
> >> Suggested-by: Chris Wilson <chris at chris-wilson.co.uk>
> >> Signed-off-by: Michal Wajdeczko <michal.wajdeczko at intel.com>
> >> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> >> Cc: Tony Ye <tony.ye at intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_huc.c | 20 ++++++++++++--------
> >>  drivers/gpu/drm/i915/intel_huc.h |  5 +++++
> >>  2 files changed, 17 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_huc.c  
> >> b/drivers/gpu/drm/i915/intel_huc.c
> >> index 1ff1fb015e58..bfdebec1cfc8 100644
> >> --- a/drivers/gpu/drm/i915/intel_huc.c
> >> +++ b/drivers/gpu/drm/i915/intel_huc.c
> >> @@ -113,7 +113,7 @@ int intel_huc_auth(struct intel_huc *huc)
> >>         u32 status;
> >>         int ret;
> >>
> >> -       if (huc->fw.load_status != INTEL_UC_FIRMWARE_SUCCESS)
> >> +       if (!intel_huc_is_loaded(huc))
> >>                 return -ENOEXEC;
> >>
> >>         ret = intel_guc_auth_huc(guc,
> >> @@ -150,21 +150,25 @@ int intel_huc_auth(struct intel_huc *huc)
> >>   * This function reads status register to verify if HuC
> >>   * firmware was successfully loaded.
> >>   *
> >> - * Returns: 1 if HuC firmware is loaded and verified,
> >> - * 0 if HuC firmware is not loaded and -ENODEV if HuC
> >> - * is not present on this platform.
> >> + * Returns: 1 if HuC firmware is loaded and verified, 0 if HuC  
> >> firmware is not
> >> + * enabled, -ENOPKG if HuC firmware is not loaded and -ENODEV if HuC  
> >> is not
> >> + * present on this platform.
> >>   */
> >>  int intel_huc_check_status(struct intel_huc *huc)
> >>  {
> >>         struct drm_i915_private *dev_priv = huc_to_i915(huc);
> >>         intel_wakeref_t wakeref;
> >> -       bool status = false;
> >> +       bool verified = false;
> >>
> >>         if (!HAS_HUC(dev_priv))
> >>                 return -ENODEV;
> >>
> >> -       with_intel_runtime_pm(dev_priv, wakeref)
> >> -               status = I915_READ(HUC_STATUS2) & HUC_FW_VERIFIED;
> >> +       if (!USES_HUC(dev_priv))
> >> +               return 0;
> >
> > Hmm, EOPNOTSUPP here for the user disabled case?
> 
> I'm not sure that user disabled case shall be reported as error,
> since it perfectly fits into legacy ABI 0="not loaded".
> The only small change is that now 0="not loaded (not enabled by user)"

The only requirement for the intel-media driver seems to "HUC_STATUS > 0".
That's our only user, so I think we have the liberty to redefine <=0 as
befits error reporting.

> 
> >
> > Then
> >
> > if (!intel_huc_is_loaded(huc))
> >       return -ENOPKG;
> >
> > bool verified = false;
> > with_intel_runtime_pm(dev_priv, wakeref)
> >       verified = I915_READ(HUC_STATUS2) & HUC_FW_VERIFIED;
> > return verified.
> >
> > That keeps it a bit flatter with the special casing separate and the 0/1
> > result as given by the register status. Then if negative, we know one of
> > the preconditions for using HuC is not available, and if 0 something went
> > wrong in mmio setup.
> >
> > Does that make sense?
> 
> IMHO, if something went wrong it is better to report it as error rather
> then weak 0 status. Compare:

I disagree, <0 is the weak case. 0 is the HW reports something went
wrong (and only that). 1 is the HW reports all went well.

If we have more ways we can report the HW went wrong, sure expand that
into extra error codes, but I don't see that. And if it's in the regs,
why are not exporting them via reg_read_ioctl?

> +---------------+-------+-------+-------+
>   no hardware     -ENODEV -ENODEV -ENODEV
>   fw disabled        0       0    -ENOTSUP
>   fw not loaded      0    -ENOPKG -ENOPKG
>   fw not verified*   0    -ENOPKG    0
>   fw verified*       1       1       1
> +---------------+-------+-------+-------+
> *) registry access
> 
> Note that in your case, fw verification problem can be reported both
> as -ENOPKG or as 0 (former when verification fails at load time, latter
> as result of runtime reg read). Very likely we will never see 0 at all,
> since probably that can't change once fw was verified at load time.

I see more distinction for ENODEV/ENOTSUP/ENOPKG/0/1. I am not getting
your point that we have conflation here, as to me it looks more
distinct, and it is clear when we report the value as given by the
register and when we give an interpreted value for a driver error.

> That might be viewed as undesired ABI change.

grep says it is fine, so I'm happy that no one else cares (as can be
seen by the lack of igt, no one has looked hard enough into how to
distinguish the API failures ;)
-Chris