Re: [PATCH v3] drm/i915: Consider HW CSB write pointer before resetting the sw read pointer

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

Michel Thierry <michel.thierry@intel.com> writes:

> A previous commit resets the Context Status Buffer (CSB) read pointer in
> ring init
>     commit c0a03a2e4c4e ("drm/i915: Reset CSB read pointer in ring init")
>
> This is generally correct, but this pointer is not reset after
> suspend/resume in some platforms (cht). In this case, the driver should
> read the register value instead of resetting the sw read counter to 0.
> Otherwise we process old events, leading to unwanted pre-emptions or
> something worse.
>
> But in other platforms (bdw) and also during GPU reset or power up, the
> CSBWP is reset to 0x7 (an invalid number), and in this case the read
> pointer should be set to 5 (the interrupt code will increment this
> counter one more time, and will start reading from CSB[0]).
>
> v2: When the CSB registers are reset, the read pointer needs to be set
> to 5, otherwise the first write (CSB[0]) won't be read (Mika).
> Replace magic numbers with GEN8_CSB_ENTRIES (6) and GEN8_CSB_PTR_MASK
> (0x07).
> v3: Rebased on top of "Parametrize LRC registers" patch.
>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> Signed-off-by: Lei Shen <lei.shen@intel.com>
> Signed-off-by: Deepak S <deepak.s@intel.com>
> Signed-off-by: Michel Thierry <michel.thierry@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c | 39 ++++++++++++++++++++++++++++++++-------
>  drivers/gpu/drm/i915/intel_lrc.h |  2 ++
>  2 files changed, 34 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 256167b..825fa7a 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -511,16 +511,16 @@ void intel_lrc_irq_handler(struct intel_engine_cs *ring)
>  	status_pointer = I915_READ(RING_CONTEXT_STATUS_PTR(ring));
>  
>  	read_pointer = ring->next_context_status_buffer;
> -	write_pointer = status_pointer & 0x07;
> +	write_pointer = status_pointer & GEN8_CSB_PTR_MASK;
>  	if (read_pointer > write_pointer)
> -		write_pointer += 6;
> +		write_pointer += GEN8_CSB_ENTRIES;
>  
>  	spin_lock(&ring->execlist_lock);
>  
>  	while (read_pointer < write_pointer) {
>  		read_pointer++;
> -		status = I915_READ(RING_CONTEXT_STATUS_BUF_LO(ring, read_pointer % 6));
> -		status_id = I915_READ(RING_CONTEXT_STATUS_BUF_HI(ring, read_pointer % 6));
> +		status = I915_READ(RING_CONTEXT_STATUS_BUF_LO(ring, read_pointer % GEN8_CSB_ENTRIES));
> +		status_id = I915_READ(RING_CONTEXT_STATUS_BUF_HI(ring, read_pointer % GEN8_CSB_ENTRIES));
>  
>  		if (status & GEN8_CTX_STATUS_IDLE_ACTIVE)
>  			continue;
> @@ -552,10 +552,12 @@ void intel_lrc_irq_handler(struct intel_engine_cs *ring)
>  	spin_unlock(&ring->execlist_lock);
>  
>  	WARN(submit_contexts > 2, "More than two context complete events?\n");
> -	ring->next_context_status_buffer = write_pointer % 6;
> +	ring->next_context_status_buffer = write_pointer % GEN8_CSB_ENTRIES;
>  
>  	I915_WRITE(RING_CONTEXT_STATUS_PTR(ring),
> -		   _MASKED_FIELD(0x07 << 8, ((u32)ring->next_context_status_buffer & 0x07) << 8));
> +		   _MASKED_FIELD(GEN8_CSB_PTR_MASK << 8,
> +				 ((u32)ring->next_context_status_buffer &
> +				  GEN8_CSB_PTR_MASK) << 8));
>  }
>  
>  static int execlists_context_queue(struct drm_i915_gem_request *request)
> @@ -1477,6 +1479,7 @@ static int gen8_init_common_ring(struct intel_engine_cs *ring)
>  {
>  	struct drm_device *dev = ring->dev;
>  	struct drm_i915_private *dev_priv = dev->dev_private;
> +	u8 next_context_status_buffer_hw;
>  
>  	lrc_setup_hardware_status_page(ring,
>  				ring->default_context->engine[ring->id].state);
> @@ -1494,7 +1497,29 @@ static int gen8_init_common_ring(struct intel_engine_cs *ring)
>  		   _MASKED_BIT_DISABLE(GFX_REPLAY_MODE) |
>  		   _MASKED_BIT_ENABLE(GFX_RUN_LIST_ENABLE));
>  	POSTING_READ(RING_MODE_GEN7(ring));
> -	ring->next_context_status_buffer = 0;
> +
> +	/*
> +	 * Instead of resetting the Context Status Buffer (CSB) read pointer to
> +	 * zero, we need to read the write pointer from hardware and use its
> +	 * value because "this register is power context save restored".
> +	 * Effectively, these states have been observed:
> +	 *
> +	 *      | Suspend-to-idle (freeze) | Suspend-to-RAM (mem) |
> +	 * BDW  | CSB regs not reset       | CSB regs reset       |
> +	 * CHT  | CSB regs not reset       | CSB regs not reset   |
> +	 */

SKL acts similar to BDW.

> +	next_context_status_buffer_hw = (I915_READ(RING_CONTEXT_STATUS_PTR(ring))
> +						   & GEN8_CSB_PTR_MASK);
> +
> +	/*
> +	 * When the CSB registers are reset (also after power-up / gpu reset),
> +	 * CSB write pointer is set to all 1's, which is not valid, use '5' in
> +	 * this special case, so the first element read is CSB[0].
> +	 */
> +	if (next_context_status_buffer_hw == GEN8_CSB_PTR_MASK)
> +		next_context_status_buffer_hw = (GEN8_CSB_ENTRIES - 1);
> +

Now that this has been unravelled, I hope we can move towards
of removing the driver state of next context buffer completely:

http://lists.freedesktop.org/archives/intel-gfx/2015-May/067313.html

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

> +	ring->next_context_status_buffer = next_context_status_buffer_hw;
>  	DRM_DEBUG_DRIVER("Execlists enabled for %s\n", ring->name);
>  
>  	memset(&ring->hangcheck, 0, sizeof(ring->hangcheck));
> diff --git a/drivers/gpu/drm/i915/intel_lrc.h b/drivers/gpu/drm/i915/intel_lrc.h
> index 8a08a27..4e60d54 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.h
> +++ b/drivers/gpu/drm/i915/intel_lrc.h
> @@ -25,6 +25,8 @@
>  #define _INTEL_LRC_H_
>  
>  #define GEN8_LR_CONTEXT_ALIGN 4096
> +#define GEN8_CSB_ENTRIES 6
> +#define GEN8_CSB_PTR_MASK 0x07
>  
>  /* Execlists regs */
>  #define RING_ELSP(ring)			((ring)->mmio_base+0x230)
> -- 
> 2.5.3



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