From 6d2cb5aa383bf020ee95e33d9d107975f340ae1c Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Wed, 13 Sep 2017 14:35:34 +0100 Subject: [PATCH] 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 Signed-off-by: Chris Wilson Cc: Michel Thierry Cc: Tvrtko Ursulin Cc: Mika Kuoppala Cc: Daniele Ceraolo Spurio Cc: Zhenyu Wang Cc: Zhi Wang Acked-by: Michel Thierry Link: https://patchwork.freedesktop.org/patch/msgid/20170913133534.26927-1-chris@chris-wilson.co.uk Reviewed-by: Mika Kuoppala --- drivers/gpu/drm/i915/i915_debugfs.c | 7 +++++-- drivers/gpu/drm/i915/intel_lrc.c | 35 ++++++++++++++++++++++++++++----- drivers/gpu/drm/i915/intel_ringbuffer.h | 3 +++ 3 files changed, 38 insertions(+), 7 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c index 6338018..7062cde 100644 --- a/drivers/gpu/drm/i915/i915_debugfs.c +++ b/drivers/gpu/drm/i915/i915_debugfs.c @@ -3315,6 +3315,7 @@ static int i915_engine_info(struct seq_file *m, void *unused) upper_32_bits(addr), lower_32_bits(addr)); if (i915.enable_execlists) { + const u32 *hws = &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX]; u32 ptr, read, write; unsigned int idx; @@ -3337,10 +3338,12 @@ static int i915_engine_info(struct seq_file *m, void *unused) write += GEN8_CSB_ENTRIES; while (read < write) { idx = ++read % GEN8_CSB_ENTRIES; - seq_printf(m, "\tExeclist CSB[%d]: 0x%08x, context: %d\n", + seq_printf(m, "\tExeclist CSB[%d]: 0x%08x [0x%08x in hwsp], context: %d [%d in hwsp]\n", idx, I915_READ(RING_CONTEXT_STATUS_BUF_LO(engine, idx)), - I915_READ(RING_CONTEXT_STATUS_BUF_HI(engine, idx))); + hws[idx * 2], + I915_READ(RING_CONTEXT_STATUS_BUF_HI(engine, idx)), + hws[idx * 2 + 1]); } rcu_read_lock(); diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index 8886e3b..6d7bbb9 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -541,10 +541,17 @@ static void intel_lrc_irq_handler(unsigned long data) while (test_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted)) { u32 __iomem *csb_mmio = dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine)); - u32 __iomem *buf = - dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)); + /* The HWSP contains a (cacheable) mirror of the CSB */ + const u32 *buf = + &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX]; unsigned int head, tail; + /* However GVT emulation depends upon intercepting CSB mmio */ + if (unlikely(engine->csb_use_mmio)) { + buf = (u32 * __force) + (dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0))); + } + /* The write will be ordered by the uncached read (itself * a memory barrier), so we do not need another in the form * of a locked instruction. The race between the interrupt @@ -584,13 +591,12 @@ static void intel_lrc_irq_handler(unsigned long data) * status notifier. */ - status = readl(buf + 2 * head); + status = READ_ONCE(buf[2 * head]); /* maybe mmio! */ if (!(status & GEN8_CTX_STATUS_COMPLETED_MASK)) continue; /* Check the context/desc id for this event matches */ - GEM_DEBUG_BUG_ON(readl(buf + 2 * head + 1) != - port->context_id); + GEM_DEBUG_BUG_ON(buf[2 * head + 1] != port->context_id); rq = port_unpack(port, &count); GEM_BUG_ON(count == 0); @@ -1720,6 +1726,23 @@ logical_ring_default_irqs(struct intel_engine_cs *engine) engine->irq_keep_mask = GT_CONTEXT_SWITCH_INTERRUPT << shift; } +static bool irq_handler_force_mmio(struct drm_i915_private *i915) +{ + /* GVT emulation depends upon intercepting CSB mmio */ + if (intel_vgpu_active(i915)) + return true; + + /* + * IOMMU adds unpredictable latency causing the CSB write (from the + * GPU into the HWSP) to only be visible some time after the interrupt + * (missed breadcrumb syndrome). + */ + if (intel_vtd_active()) + return true; + + return false; +} + static void logical_ring_setup(struct intel_engine_cs *engine) { @@ -1731,6 +1754,8 @@ logical_ring_setup(struct intel_engine_cs *engine) /* Intentionally left blank. */ engine->buffer = NULL; + engine->csb_use_mmio = irq_handler_force_mmio(dev_priv); + fw_domains = intel_uncore_forcewake_for_reg(dev_priv, RING_ELSP(engine), FW_REG_WRITE); diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h index 79c0021..5c055b6 100644 --- a/drivers/gpu/drm/i915/intel_ringbuffer.h +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h @@ -391,6 +391,7 @@ struct intel_engine_cs { struct rb_root execlist_queue; struct rb_node *execlist_first; unsigned int fw_domains; + bool csb_use_mmio; /* Contexts are pinned whilst they are active on the GPU. The last * context executed remains active whilst the GPU is idle - the @@ -496,6 +497,8 @@ intel_write_status_page(struct intel_engine_cs *engine, int reg, u32 value) #define I915_GEM_HWS_SCRATCH_INDEX 0x40 #define I915_GEM_HWS_SCRATCH_ADDR (I915_GEM_HWS_SCRATCH_INDEX << MI_STORE_DWORD_INDEX_SHIFT) +#define I915_HWS_CSB_BUF0_INDEX 0x10 + struct intel_ring * intel_engine_create_ring(struct intel_engine_cs *engine, int size); int intel_ring_pin(struct intel_ring *ring, -- 2.7.4