[Intel-gfx] [PATCH] drm/i915/mtl: Check full IP version when applying hw steering semaphore

[Matt Roper]

On Mon, Dec 05, 2022 at 12:50:40PM +0000, Tvrtko Ursulin wrote:
> 
> On 02/12/2022 22:49, Rodrigo Vivi wrote:
> > On Fri, Dec 02, 2022 at 02:35:28PM -0800, Matt Roper wrote:
> > > When determining whether the platform has a hardware-level steering
> > > semaphore (i.e., MTL and beyond), we need to use GRAPHICS_VER_FULL() to
> > > compare the full version rather than just the major version number
> > > returned by GRAPHICS_VER().
> > > 
> > > Reported-by: kernel test robot <lkp at intel.com>
> > > Fixes: 3100240bf846 ("drm/i915/mtl: Add hardware-level lock for steering")
> > > Cc: Balasubramani Vivekanandan <balasubramani.vivekanandan at intel.com>
> > > Signed-off-by: Matt Roper <matthew.d.roper at intel.com>
> > 
> > Reviewed-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> > > ---
> > >   drivers/gpu/drm/i915/gt/intel_gt_mcr.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/gt/intel_gt_mcr.c b/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
> > > index 087e4ac5b68d..41a237509dcf 100644
> > > --- a/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
> > > +++ b/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
> > > @@ -367,7 +367,7 @@ void intel_gt_mcr_lock(struct intel_gt *gt, unsigned long *flags)
> > >   	 * driver threads, but also with hardware/firmware agents.  A dedicated
> > >   	 * locking register is used.
> > >   	 */
> > > -	if (GRAPHICS_VER(gt->i915) >= IP_VER(12, 70))
> > > +	if (GRAPHICS_VER_FULL(gt->i915) >= IP_VER(12, 70))
> 
> Ouch, tricky class of bugs... Anyone has an idea how to maybe coerce the compiler into spotting them for us, cheaply?

I believe clang can already notice these problems with
Wtautological-constant-out-of-range-compare (which is how the kernel
test robot finds them):

	>> drivers/gpu/drm/i915/gt/intel_gt_mcr.c:370:29: warning: result of comparison of constant 3142 with expression of type 'u8' (aka 'unsigned char')
	+is always false [-Wtautological-constant-out-of-range-compare]
		   if (GRAPHICS_VER(gt->i915) >= IP_VER(12, 70))
		       ~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~
	   drivers/gpu/drm/i915/gt/intel_gt_mcr.c:410:29: warning: result of comparison of constant 3142 with expression of type 'u8' (aka 'unsigned char')
	+is always false [-Wtautological-constant-out-of-range-compare]
		   if (GRAPHICS_VER(gt->i915) >= IP_VER(12, 70))
		       ~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~
	   2 warnings generated.

Unfortunately gcc doesn't seem to have anything equivalent as far as I
can see.

> 
> This one is undefined behaviour I think so not good:
> 
> -#define IP_VER(ver, rel)               ((ver) << 8 | (rel))
> +typedef void * i915_full_ver_t;
> +
> +#define IP_VER(ver, rel) (i915_full_ver_t)(unsigned long)((ver) << 8 | (rel))

Hmm, so by casting it into a pointer, you're hoping to trigger a
"comparison of pointer and integer without cast" warning on misuse?
That's a good idea, but as you noted, the C99 spec says comparison of
pointers is only guaranteed to work if both are pointers into the same
structure/array, otherwise the results are technically undefined.


Matt

> 
> Regards,
> 
> Tvrtko
> 
> > >   		err = wait_for(intel_uncore_read_fw(gt->uncore,
> > >   						    MTL_STEER_SEMAPHORE) == 0x1, 100);
> > > @@ -407,7 +407,7 @@ void intel_gt_mcr_unlock(struct intel_gt *gt, unsigned long flags)
> > >   {
> > >   	spin_unlock_irqrestore(&gt->mcr_lock, flags);
> > > -	if (GRAPHICS_VER(gt->i915) >= IP_VER(12, 70))
> > > +	if (GRAPHICS_VER_FULL(gt->i915) >= IP_VER(12, 70))
> > >   		intel_uncore_write_fw(gt->uncore, MTL_STEER_SEMAPHORE, 0x1);
> > >   }
> > > -- 
> > > 2.38.1
> > > 

-- 
Matt Roper
Graphics Software Engineer
VTT-OSGC Platform Enablement
Intel Corporation