Re: [Intel-gfx] [PATCH v2 4/5] drm/i915/mtl: Add hardware-level lock for steering

Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx> · Mon, 5 Dec 2022 08:58:16 +0000

On 28/11/2022 23:30, Matt Roper wrote:
Starting with MTL, the driver needs to not only protect the steering
control register from simultaneous software accesses, but also protect
against races with hardware/firmware agents.  The hardware provides a
dedicated locking mechanism to support this via the MTL_STEER_SEMAPHORE
register.  Reading the register acts as a 'trylock' operation; the read
will return 0x1 if the lock is acquired or 0x0 if something else is
already holding the lock; once acquired, writing 0x1 to the register
will release the lock.

We'll continue to grab the software lock as well, just so lockdep can
track our locking; assuming the hardware lock is behaving properly,
there should never be any contention on the software lock in this case.

v2:
  - Extend hardware semaphore timeout and add a taint for CI if it ever
    happens (this would imply misbehaving hardware/firmware).  (Mika)
  - Add "MTL_" prefix to new steering semaphore register.  (Mika)

Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
Signed-off-by: Matt Roper <matthew.d.roper@xxxxxxxxx>
---
  drivers/gpu/drm/i915/gt/intel_gt_mcr.c  | 38 ++++++++++++++++++++++---
  drivers/gpu/drm/i915/gt/intel_gt_regs.h |  1 +
  2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_mcr.c b/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
index aa070ae57f11..087e4ac5b68d 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_mcr.c
@@ -347,10 +347,9 @@ static u32 rw_with_mcr_steering(struct intel_gt *gt,
   * @flags: storage to save IRQ flags to
   *
   * Performs locking to protect the steering for the duration of an MCR
- * operation.  Depending on the platform, this may be a software lock
- * (gt->mcr_lock) or a hardware lock (i.e., a register that synchronizes
- * access not only for the driver, but also for external hardware and
- * firmware agents).
+ * operation.  On MTL and beyond, a hardware lock will also be taken to
+ * serialize access not only for the driver, but also for external hardware and
+ * firmware agents.
   *
   * Context: Takes gt->mcr_lock.  uncore->lock should *not* be held when this
   *          function is called, although it may be acquired after this
@@ -359,12 +358,40 @@ static u32 rw_with_mcr_steering(struct intel_gt *gt,
  void intel_gt_mcr_lock(struct intel_gt *gt, unsigned long *flags)
  {
  	unsigned long __flags;
+	int err = 0;
  
  	lockdep_assert_not_held(&gt->uncore->lock);
  
+	/*
+	 * Starting with MTL, we need to coordinate not only with other
+	 * driver threads, but also with hardware/firmware agents.  A dedicated
+	 * locking register is used.
+	 */
+	if (GRAPHICS_VER(gt->i915) >= IP_VER(12, 70))
+		err = wait_for(intel_uncore_read_fw(gt->uncore,
+						    MTL_STEER_SEMAPHORE) == 0x1, 100);
+

If two i915 threads enter here what happens? (Given hw locking is done 
before the spinlock.)

Regards,

Tvrtko

+	/*
+	 * Even on platforms with a hardware lock, we'll continue to grab
+	 * a software spinlock too for lockdep purposes.  If the hardware lock
+	 * was already acquired, there should never be contention on the
+	 * software lock.
+	 */
  	spin_lock_irqsave(&gt->mcr_lock, __flags);
  
  	*flags = __flags;
+
+	/*
+	 * In theory we should never fail to acquire the HW semaphore; this
+	 * would indicate some hardware/firmware is misbehaving and not
+	 * releasing it properly.
+	 */
+	if (err == -ETIMEDOUT) {
+		drm_err_ratelimited(&gt->i915->drm,
+				    "GT%u hardware MCR steering semaphore timed out",
+				    gt->info.id);
+		add_taint_for_CI(gt->i915, TAINT_WARN);  /* CI is now unreliable */
+	}
  }
  
  /**
@@ -379,6 +406,9 @@ void intel_gt_mcr_lock(struct intel_gt *gt, unsigned long *flags)
  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))
+		intel_uncore_write_fw(gt->uncore, MTL_STEER_SEMAPHORE, 0x1);
  }
  
  /**
diff --git a/drivers/gpu/drm/i915/gt/intel_gt_regs.h b/drivers/gpu/drm/i915/gt/intel_gt_regs.h
index 784152548472..1618d46cb8c7 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_regs.h
+++ b/drivers/gpu/drm/i915/gt/intel_gt_regs.h
@@ -67,6 +67,7 @@
  #define GMD_ID_MEDIA				_MMIO(MTL_MEDIA_GSI_BASE + 0xd8c)
  
  #define MCFG_MCR_SELECTOR			_MMIO(0xfd0)
+#define MTL_STEER_SEMAPHORE			_MMIO(0xfd0)
  #define MTL_MCR_SELECTOR			_MMIO(0xfd4)
  #define SF_MCR_SELECTOR				_MMIO(0xfd8)
  #define GEN8_MCR_SELECTOR			_MMIO(0xfdc)






