Re: [PATCH 6/6] drm/i915/execlists: Read the context-status HEAD from the HWSP

Mika Kuoppala
2017-09-13T14:14:33Z

Chris Wilson <chris@chris-wilson.co.uk> writes:

> The engine also provides a mirror of the CSB write pointer in the HWSP,
> but not of our read pointer. To take advantage of this we need to
> remember where we read up to on the last interrupt and continue off from
> there. This poses a problem following a reset, as we don't know where
> the hw will start writing from, and due to the use of power contexts we
> cannot perform that query during the reset itself. So we continue the
> current modus operandi of delaying the first read of the context-status
> read/write pointers until after the first interrupt. With this we should
> now have eliminated all uncached mmio reads in handling the
> context-status interrupt, though we still have the uncached mmio writes
> for submitting new work, and many uncached mmio reads in the global
> interrupt handler itself. Still a step in the right direction towards
> reducing our resubmit latency, although it appears lost in the noise!
>
> v2: Cannonlake moved the CSB write index
> v3: Include the sw/hwsp state in debugfs/i915_engine_info
> v4: Also revert to using CSB mmio for GVT-g
> v5: Prevent the compiler reloading tail (Mika)
>
> 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>
> ---
> drivers/gpu/drm/i915/i915_debugfs.c | 6 ++++--
> drivers/gpu/drm/i915/i915_drv.h | 8 ++++++++
> drivers/gpu/drm/i915/intel_lrc.c | 27 ++++++++++++++++++++-------
> drivers/gpu/drm/i915/intel_ringbuffer.h | 3 +++
> 4 files changed, 35 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
> index 7062cde94a49..12381045ed6a 100644
> --- a/drivers/gpu/drm/i915/i915_debugfs.c
> +++ b/drivers/gpu/drm/i915/i915_debugfs.c
> @@ -3326,8 +3326,10 @@ static int i915_engine_info(struct seq_file *m, void *unused)
> ptr = I915_READ(RING_CONTEXT_STATUS_PTR(engine));
> read = GEN8_CSB_READ_PTR(ptr);
> write = GEN8_CSB_WRITE_PTR(ptr);
> - seq_printf(m, "\tExeclist CSB read %d, write %d, interrupt posted? %s\n",
> - read, write,
> + seq_printf(m, "\tExeclist CSB read %d [%d cached], write %d [%d from hws], interrupt posted? %s\n",
> + read, engine->csb_head,
> + write,
> + intel_read_status_page(engine, intel_hws_csb_write_index(engine->i915)),
> yesno(test_bit(ENGINE_IRQ_EXECLIST,
> &engine->irq_posted)));
> if (read >= GEN8_CSB_ENTRIES)
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 1cc31a5b049f..78195b7c64db 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -4399,4 +4399,12 @@ int remap_io_mapping(struct vm_area_struct *vma,
> unsigned long addr, unsigned long pfn, unsigned long size,
> struct io_mapping *iomap);
>
> +static inline int intel_hws_csb_write_index(struct drm_i915_private *i915)
> +{
> + if (INTEL_GEN(i915) >= 10)
> + return CNL_HWS_CSB_WRITE_INDEX;
> + else
> + return I915_HWS_CSB_WRITE_INDEX;
> +}
> +
> #endif
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 8e4b21a18554..55d7eee21226 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -545,8 +545,6 @@ static void intel_lrc_irq_handler(unsigned long data)
> * new request (outside of the context-switch interrupt).
> */
> 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));
> /* The HWSP contains a (cacheable) mirror of the CSB */
> const u32 *buf =
> &engine->status_page.page_addr[I915_HWS_CSB_BUF0_INDEX];
> @@ -556,6 +554,7 @@ static void intel_lrc_irq_handler(unsigned long data)
> if (unlikely(engine->csb_use_mmio)) {
> buf = (u32 * __force)
> (dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_BUF_LO(engine, 0)));
> + engine->csb_head = -1; /* force mmio read of CSB ptrs */
> }
>
> /* The write will be ordered by the uncached read (itself
> @@ -569,9 +568,19 @@ static void intel_lrc_irq_handler(unsigned long data)
> * is set and we do a new loop.
> */
> __clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
> - head = readl(csb_mmio);
> - tail = GEN8_CSB_WRITE_PTR(head);
> - head = GEN8_CSB_READ_PTR(head);
> + if (unlikely(engine->csb_head == -1)) { /* following a reset */

Was going to suggest using the same csb_use_mmio flag for this
but that would not gain much when looking at the read ptr write
further down.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

> + head = readl(dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine)));
> + tail = GEN8_CSB_WRITE_PTR(head);
> + head = GEN8_CSB_READ_PTR(head);
> + engine->csb_head = head;
> + } else {
> + const int write_idx =
> + intel_hws_csb_write_index(dev_priv) -
> + I915_HWS_CSB_BUF0_INDEX;
> +
> + head = engine->csb_head;
> + tail = READ_ONCE(buf[write_idx]);
> + }
> while (head != tail) {
> struct drm_i915_gem_request *rq;
> unsigned int status;
> @@ -625,8 +634,11 @@ static void intel_lrc_irq_handler(unsigned long data)
> !(status & GEN8_CTX_STATUS_ACTIVE_IDLE));
> }
>
> - writel(_MASKED_FIELD(GEN8_CSB_READ_PTR_MASK, head << 8),
> - csb_mmio);
> + if (head != engine->csb_head) {
> + engine->csb_head = head;
> + writel(_MASKED_FIELD(GEN8_CSB_READ_PTR_MASK, head << 8),
> + dev_priv->regs + i915_mmio_reg_offset(RING_CONTEXT_STATUS_PTR(engine)));
> + }
> }
>
> if (execlists_elsp_ready(engine))
> @@ -1275,6 +1287,7 @@ static int gen8_init_common_ring(struct intel_engine_cs *engine)
> I915_WRITE(GEN8_GT_IIR(gtiir[engine->id]),
> GT_CONTEXT_SWITCH_INTERRUPT << engine->irq_shift);
> clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
> + engine->csb_head = -1;
>
> /* After a GPU reset, we may have requests to replay */
> submit = false;
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 5c055b62966d..4d63a2c0b2e1 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -392,6 +392,7 @@ struct intel_engine_cs {
> struct rb_node *execlist_first;
> unsigned int fw_domains;
> bool csb_use_mmio;
> + unsigned int csb_head;
>
> /* Contexts are pinned whilst they are active on the GPU. The last
> * context executed remains active whilst the GPU is idle - the
> @@ -498,6 +499,8 @@ intel_write_status_page(struct intel_engine_cs *engine, int reg, u32 value)
> #define I915_GEM_HWS_SCRATCH_ADDR (I915_GEM_HWS_SCRATCH_INDEX << MI_STORE_DWORD_INDEX_SHIFT)
>
> #define I915_HWS_CSB_BUF0_INDEX 0x10
> +#define I915_HWS_CSB_WRITE_INDEX 0x1f
> +#define CNL_HWS_CSB_WRITE_INDEX 0x2f
>
> struct intel_ring *
> intel_engine_create_ring(struct intel_engine_cs *engine, int size);
> --
> 2.14.1