Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> 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
>
> Suggested-by: Ben Widawsky <benjamin.widawsky@xxxxxxxxx>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Michel Thierry <michel.thierry@xxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@xxxxxxxxx>
> Cc: Zhenyu Wang <zhenyuw@xxxxxxxxxxxxxxx>
> Cc: Zhi Wang <zhi.a.wang@xxxxxxxxx>
> Acked-by: Michel Thierry <michel.thierry@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_debugfs.c | 7 +++++--
> drivers/gpu/drm/i915/intel_lrc.c | 16 +++++++++++-----
> drivers/gpu/drm/i915/intel_ringbuffer.h | 2 ++
> 3 files changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 620c9218d1c1..5fd01c14a3ec 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -3384,6 +3384,7 @@ 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;
>
> @@ -3404,10 +3405,12 @@ 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 3469badedbe0..41dc04eb6097 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -547,10 +547,17 @@ 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];
Could be also const u32 * const buf = ...
as in debugfs counterpart. Value added is quite thin tho vs clutter so
not insisting.
> unsigned int head, tail;
>
> + /* However GVT emulation depends upon intercepting CSB mmio */
> + if (unlikely(intel_vgpu_active(dev_priv))) {
> + 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
> @@ -590,13 +597,12 @@ static void intel_lrc_irq_handler(unsigned long data)
> * status notifier.
> */
>
> - status = readl(buf + 2 * head);
> + status = buf[2 * head];
> 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);
In here I wonder if GEM_BUG_ON check with the equivalence of the hswp value
vs mmio valu would serve any purpose. Adding the mmio delay here tho
would be harmful.
Reviewed-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
>
> rq = port_unpack(port, &count);
> GEM_BUG_ON(count == 0);
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index d33c93444c0d..2c55cfa14fb5 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -496,6 +496,8 @@ 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,
> --
> 2.13.2
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