Re: [PATCH v3 05/16] drm/i915: Disable the "binder"

[Nirmoy Das <nirmoy.das@xxxxxxxxx>]

On 1/19/2024 11:47 AM, Nirmoy Das wrote:

On 1/19/2024 12:12 AM, Ville Syrjälä wrote:

On Wed, Jan 17, 2024 at 06:46:24PM +0100, Nirmoy Das wrote:

On 1/17/2024 3:13 PM, Michał Winiarski wrote:

On Tue, Jan 16, 2024 at 09:56:25AM +0200, Ville Syrjala wrote:

From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>

Now that the GGTT PTE updates go straight to GSMBASE (bypassing
GTTMMADR) there should be no more risk of system hangs? So the
"binder" (ie. update the PTEs via MI_UPDATE_GTT) is no longer
necessary, disable it.

My main worry with the MI_UPDATE_GTT are:
- only used on this one platform so very limited testing coverage
- async so more opprtunities to screw things up
- what happens if the engine hangs while we're waiting for MI_UPDATE_GTT
   to finish?
- requires working command submission, so even getting a working
   display now depends on a lot more extra components working correctly

TODO: MI_UPDATE_GTT might be interesting as an optimization
though, so perhaps someone should look into always using it
(assuming the GPU is alive and well)?

v2: Keep using MI_UPDATE_GTT on VM guests

Cc: Paz Zcharya <pazz@xxxxxxxxxxxx>
Cc: Nirmoy Das <nirmoy.das@xxxxxxxxx>
Signed-off-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
---
  drivers/gpu/drm/i915/gt/intel_gtt.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gtt.c b/drivers/gpu/drm/i915/gt/intel_gtt.c
index 86f73fe558ca..e83dabc56a14 100644
--- a/drivers/gpu/drm/i915/gt/intel_gtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_gtt.c
@@ -24,7 +24,8 @@
  bool i915_ggtt_require_binder(struct drm_i915_private *i915)
  {
  	/* Wa_13010847436 & Wa_14019519902 */
-	return MEDIA_VER_FULL(i915) == IP_VER(13, 0);
+	return i915_run_as_guest() &&
+		MEDIA_VER_FULL(i915) == IP_VER(13, 0);

Note that i915_run_as_guest() is not the most reliable way to decide
whether to use MI_UPDATE_GTT or straight to GSMBASE, as it requires the
hypervisor to "opt-in" and set the X86_FEATURE_HYPERVISOR.
If it's not set - the driver will go into GSMBASE, which is not mapped
inside the guest.
Does the system firmware advertise whether GSMBASE is "open" or "closed"
to CPU access in any way?

Had a chat with David from IVE team, David suggested to read 0x138914 to 
determine that.  "GOP needs to qualify the WA by reading GFX MMIO offset 
0x138914 and verify the value there is 0x1." -> as per the HSD-22018444074

OK, so we can confirm the firmware is on board. I suppose no real harm
in doing so even though it would clearly be a rather weird if someone
would ship some ancient firmware that doesn't handle this.

But that still won't help with the guest side handling because that
register will read the same in the guest.

We are back to the same question :/ How about
if (boot_cpu_has(X86_FEATURE_HYPERVISOR) && !i915_run_as_guest()

hmm, never mind that was stupid.

disable binder

Regards,

Nirmoy