Re: [PATCH] drm/i915: Add support for resetting the SO write pointers on gen7.

[Ben Widawsky <ben@bwidawsk.net>]

On 12/29/2011 05:52 PM, Eric Anholt wrote:
> These registers are automatically incremented by the hardware during
> transform feedback to track where the next streamed vertex output
> should go.  Unlike the previous generation, which had a packet for
> setting the corresponding registers to a defined value, gen7 only has
> MI_LOAD_REGISTER_IMM to do so.  That's a secure packet (since it loads
> an arbitrary register), so we need to do it from the kernel, and it
> needs to be settable atomically with the batchbuffer execution so that
> two clients doing transform feedback don't stomp on each others'
> state.
>
> Instead of building a more complicated interface involcing setting the
> registers to a specific value, just set them to 0 when asked and
> userland can tweak its pointers accordingly.
>
> Signed-off-by: Eric Anholt<eric@anholt.net>
> ---
>   drivers/gpu/drm/i915/i915_dma.c            |    3 ++
>   drivers/gpu/drm/i915/i915_gem_execbuffer.c |   31 ++++++++++++++++++++++++++++
>   drivers/gpu/drm/i915/i915_reg.h            |    6 +++++
>   include/drm/i915_drm.h                     |    4 +++
>   4 files changed, 44 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
> index a9ae374..1add685 100644
> --- a/drivers/gpu/drm/i915/i915_dma.c
> +++ b/drivers/gpu/drm/i915/i915_dma.c
> @@ -781,6 +781,9 @@ static int i915_getparam(struct drm_device *dev, void *data,
>   	case I915_PARAM_HAS_RELAXED_DELTA:
>   		value = 1;
>   		break;
> +	case I915_PARAM_HAS_GEN7_SOL_RESET:
> +		value = 1;
> +		break;
>   	default:
>   		DRM_DEBUG_DRIVER("Unknown parameter %d\n",
>   				 param->param);
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index a4e4f3a..126144a 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -971,6 +971,31 @@ i915_gem_execbuffer_retire_commands(struct drm_device *dev,
>   }
>
>   static int
> +i915_reset_gen7_sol_offsets(struct drm_device *dev,
> +			    struct intel_ring_buffer *ring)
> +{
> +	drm_i915_private_t *dev_priv = dev->dev_private;
> +	int ret, i;
> +
> +	if (!IS_GEN7(dev) || ring !=&dev_priv->ring[RCS])
> +		return 0;
> +
> +	ret = intel_ring_begin(ring, 4 * 3);
> +	if (ret)
> +		return ret;
> +
> +	for (i = 0; i<  4; i++) {
> +		intel_ring_emit(ring, MI_LOAD_REGISTER_IMM(1));
> +		intel_ring_emit(ring, GEN7_SO_WRITE_OFFSET(i));
> +		intel_ring_emit(ring, 0);
> +	}
> +
> +	intel_ring_advance(ring);
> +
> +	return 0;
> +}
> +
> +static int
>   i915_gem_do_execbuffer(struct drm_device *dev, void *data,
>   		       struct drm_file *file,
>   		       struct drm_i915_gem_execbuffer2 *args,
> @@ -1182,6 +1207,12 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
>   		goto err;
>   	}
>
> +	if (args->flags&  I915_EXEC_GEN7_SOL_RESET) {
> +		ret = i915_reset_gen7_sol_offsets(dev, ring);
> +		if (ret)
> +			goto err;
> +	}
> +
>   	trace_i915_gem_ring_dispatch(ring, seqno);
>
>   	exec_start = batch_obj->gtt_offset + args->batch_start_offset;
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 9d15474..54a18a4 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -3583,4 +3583,10 @@
>   #define GEN7_AUD_CNTRL_ST_A		0xE50B4
>   #define GEN7_AUD_CNTRL_ST2		0xE50C0
>
> +/* These are the 4 32-bit write offset registers for each stream
> + * output buffer.  It determines the offset from the
> + * 3DSTATE_SO_BUFFERs that the next streamed vertex output goes to.
> + */
> +#define GEN7_SO_WRITE_OFFSET(n)		(0x5280 + (n) * 4)
> +
>   #endif /* _I915_REG_H_ */
> diff --git a/include/drm/i915_drm.h b/include/drm/i915_drm.h
> index 28c0d11..5bb6a6a 100644
> --- a/include/drm/i915_drm.h
> +++ b/include/drm/i915_drm.h
> @@ -291,6 +291,7 @@ typedef struct drm_i915_irq_wait {
>   #define I915_PARAM_HAS_COHERENT_RINGS	 13
>   #define I915_PARAM_HAS_EXEC_CONSTANTS	 14
>   #define I915_PARAM_HAS_RELAXED_DELTA	 15
> +#define I915_PARAM_HAS_GEN7_SOL_RESET	 16
>
>   typedef struct drm_i915_getparam {
>   	int param;
> @@ -653,6 +654,9 @@ struct drm_i915_gem_execbuffer2 {
>   	__u64 rsvd2;
>   };
>
> +/** Resets the SO write offset registers for transform feedback on gen7. */
> +#define I915_EXEC_GEN7_SOL_RESET	(1<<8)
> +
>   struct drm_i915_gem_pin {
>   	/** Handle of the buffer to be pinned. */
>   	__u32 handle;

Is this something we want to carry long term wrt ABI. I really want to 
get context/ppgtt stuff out the door relatively soon (context should be 
ready to test really soon actually).

I'm totally unfamiliar with this register, but is this what you want the 
interface to be permanently?

Ben