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drm/i915/execlists: Read the context-status buffer from the HWSP
The engine provides a mirror of the CSB in the HWSP. If we use the cacheable reads from the HWSP, we can shave off a few mmio reads per context-switch interrupt (which are quite frequent!). Just removing a couple of mmio is not enough to actually reduce any latency, but a small reduction in overall cpu usage. Much appreciation for Ben dropping the bombshell that the CSB was in the HWSP and for Michel in digging out the details. v2: Don't be lazy, add the defines for the indices. v3: Include the HWSP in debugfs/i915_engine_info v4: Check for GVT-g, it currently depends on intercepting CSB mmio v5: Fixup GVT-g mmio path v6: Disable HWSP if VT-d is active as the iommu adds unpredictable memory latency. (Mika) v7: Also markup the CSB read with READ_ONCE() as it may still be an mmio read and we want to stop the compiler from issuing a later (v.slow) reload. Suggested-by: Ben Widawsky <benjamin.widawsky@intel.com> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Michel Thierry <michel.thierry@intel.com> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: Mika Kuoppala <mika.kuoppala@intel.com> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com> Cc: Zhenyu Wang <zhenyuw@linux.intel.com> Cc: Zhi Wang <zhi.a.wang@intel.com> Acked-by: Michel Thierry <michel.thierry@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20170913133534.26927-1-chris@chris-wilson.co.uk Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
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parent
34a04e5e46
commit
6d2cb5aa38
3 changed files with 38 additions and 7 deletions
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@ -3315,6 +3315,7 @@ static int i915_engine_info(struct seq_file *m, void *unused)
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upper_32_bits(addr), lower_32_bits(addr));
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if (i915.enable_execlists) {
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const u32 *hws = &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
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u32 ptr, read, write;
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unsigned int idx;
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@ -3337,10 +3338,12 @@ static int i915_engine_info(struct seq_file *m, void *unused)
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write += GEN8_CSB_ENTRIES;
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while (read < write) {
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idx = ++read % GEN8_CSB_ENTRIES;
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seq_printf(m, "\tExeclist CSB[%d]: 0x%08x, context: %d\n",
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seq_printf(m, "\tExeclist CSB[%d]: 0x%08x [0x%08x in hwsp], context: %d [%d in hwsp]\n",
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idx,
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I915_READ(RING_CONTEXT_STATUS_BUF_LO(engine, idx)),
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I915_READ(RING_CONTEXT_STATUS_BUF_HI(engine, idx)));
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hws[idx * 2],
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I915_READ(RING_CONTEXT_STATUS_BUF_HI(engine, idx)),
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hws[idx * 2 + 1]);
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}
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rcu_read_lock();
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@ -541,10 +541,17 @@ static void intel_lrc_irq_handler(unsigned long data)
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while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) {
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u32 __iomem *csb_mmio =
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dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine));
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u32 __iomem *buf =
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dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0));
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/* The HWSP contains a (cacheable) mirror of the CSB */
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const u32 *buf =
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&engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
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unsigned int head, tail;
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/* However GVT emulation depends upon intercepting CSB mmio */
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if (unlikely(engine->csb_use_mmio)) {
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buf = (u32 * __force)
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(dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)));
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}
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/* The write will be ordered by the uncached read (itself
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* a memory barrier), so we do not need another in the form
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* of a locked instruction. The race between the interrupt
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@ -584,13 +591,12 @@ static void intel_lrc_irq_handler(unsigned long data)
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* status notifier.
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*/
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status = readl(buf + 2 * head);
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status = READ_ONCE(buf[2 * head]); /* maybe mmio! */
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if (!(status & GEN8_CTX_STATUS_COMPLETED_MASK))
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continue;
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/* Check the context/desc id for this event matches */
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GEM_DEBUG_BUG_ON(readl(buf + 2 * head + 1) !=
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port->context_id);
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GEM_DEBUG_BUG_ON(buf[2 * head + 1] != port->context_id);
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rq = port_unpack(port, &count);
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GEM_BUG_ON(count == 0);
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@ -1720,6 +1726,23 @@ logical_ring_default_irqs(struct intel_engine_cs *engine)
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engine->irq_keep_mask = GT_CONTEXT_SWITCH_INTERRUPT << shift;
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}
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static bool irq_handler_force_mmio(struct drm_i915_private *i915)
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{
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/* GVT emulation depends upon intercepting CSB mmio */
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if (intel_vgpu_active(i915))
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return true;
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/*
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* IOMMU adds unpredictable latency causing the CSB write (from the
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* GPU into the HWSP) to only be visible some time after the interrupt
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* (missed breadcrumb syndrome).
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*/
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if (intel_vtd_active())
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return true;
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return false;
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}
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static void
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logical_ring_setup(struct intel_engine_cs *engine)
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{
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@ -1731,6 +1754,8 @@ logical_ring_setup(struct intel_engine_cs *engine)
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/* Intentionally left blank. */
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engine->buffer = NULL;
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engine->csb_use_mmio = irq_handler_force_mmio(dev_priv);
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fw_domains = intel_uncore_forcewake_for_reg(dev_priv,
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RING_ELSP(engine),
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FW_REG_WRITE);
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@ -391,6 +391,7 @@ struct intel_engine_cs {
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struct rb_root execlist_queue;
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struct rb_node *execlist_first;
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unsigned int fw_domains;
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bool csb_use_mmio;
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/* Contexts are pinned whilst they are active on the GPU. The last
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* context executed remains active whilst the GPU is idle - the
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@ -496,6 +497,8 @@ intel_write_status_page(struct intel_engine_cs *engine, int reg, u32 value)
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#define I915_GEM_HWS_SCRATCH_INDEX 0x40
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#define I915_GEM_HWS_SCRATCH_ADDR (I915_GEM_HWS_SCRATCH_INDEX << MI_STORE_DWORD_INDEX_SHIFT)
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#define I915_HWS_CSB_BUF0_INDEX 0x10
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struct intel_ring *
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intel_engine_create_ring(struct intel_engine_cs *engine, int size);
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int intel_ring_pin(struct intel_ring *ring,
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