[Intel-gfx] [PATCH] drm/i915: Steer MCR reads to lowest potential slice/subslice

[Matt Roper]

When determining the slice/subslice to use for steering multicast
register reads we need to not only consider the fusing (to avoid
steering to a fused off instance), but also the "minconfig" that the
hardware uses to wake from RC6 when render power gating is enabled on
some platforms.  Although it isn't well-documented, certain platforms
(e.g., EHL/JSL) will initially only power up a single instance of
multicast registers when forcewake is grabbed if the GPU isn't actually
busy.  In these cases, only the minconfig slice/subslice will return
valid, non-zero reads so we need to ensure that we steer to the
minconfig specifically.  The minconfig should always be the lowest
slice/subslice that isn't fused off; as such, we should pick our
steering target with ffs() instead of fls() during initialization.

This steering change is especially important on EHL/JSL since there are
cases where the hardware appears to not honor the driver's attempts to
disable render powergating via the POWERGATE_ENABLE (0xA210) register
and will continue to only wake the minconfig's slice/subslice in
response to forcewake.  We can see this in certain reset or
suspend/resume cases where i915 tries to disable render powergating and
then re-applies workarounds before re-enabling powergating; the
workarounds apply successfully, but the readback verification will fail
if we aren't steering to the minconfig register instance.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/1222
Cc: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay at intel.com>
Signed-off-by: Matt Roper <matthew.d.roper at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_workarounds.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
index b62d1e31a645..0c973678bf03 100644
--- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
+++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
@@ -991,13 +991,13 @@ wa_init_mcr(struct drm_i915_private *i915, struct i915_wa_list *wal)
 		l3_en = ~0;
 	}
 
-	slice = fls(sseu->slice_mask) - 1;
-	subslice = fls(l3_en & intel_sseu_get_subslices(sseu, slice));
+	slice = ffs(sseu->slice_mask) - 1;
+	subslice = ffs(l3_en & intel_sseu_get_subslices(sseu, slice));
 	if (!subslice) {
 		drm_warn(&i915->drm,
 			 "No common index found between subslice mask %x and L3 bank mask %x!\n",
 			 intel_sseu_get_subslices(sseu, slice), l3_en);
-		subslice = fls(l3_en);
+		subslice = ffs(l3_en);
 		drm_WARN_ON(&i915->drm, !subslice);
 	}
 	subslice--;
-- 
2.25.4