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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>
This commit is contained in:
Chris Wilson 2017-09-13 14:35:34 +01:00
parent 34a04e5e46
commit 6d2cb5aa38
3 changed files with 38 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
drivers/gpu/drm/i915/i915_debugfs.c
View File

@ -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();

35
drivers/gpu/drm/i915/intel_lrc.c
View File

@ -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);

3
drivers/gpu/drm/i915/intel_ringbuffer.h
View File

@ -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,