Re: [PATCH] drm/i915: Drop support for I915_EXEC_CONSTANTS_* execbuf parameters.

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On Wed, Feb 15, 2017 at 01:34:46AM -0800, Kenneth Graunke wrote:
> This patch makes the I915_PARAM_HAS_EXEC_CONSTANTS getparam return 0
> (indicating the optional feature is not supported), and makes execbuf
> always return -EINVAL if the flags are used.
> 
> Apparently, no userspace ever shipped which used this optional feature:
> I checked the git history of Mesa, xf86-video-intel, libva, and Beignet,
> and there were zero commits showing a use of these flags.  Kernel commit
> 72bfa19c8deb4 apparently introduced the feature prematurely.  According
> to Chris, the intention was to use this in cairo-drm, but "the use was
> broken for gen6", so I don't think it ever happened.
> 
> 'relative_constants_mode' has always been tracked per-device, but this
> has actually been wrong ever since hardware contexts were introduced, as
> the INSTPM register is saved (and automatically restored) as part of the
> render ring context. The software per-device value could therefore get
> out of sync with the hardware per-context value.  This meant that using
> them is actually unsafe: a client which tried to use them could damage
> the state of other clients, causing the GPU to interpret their BO
> offsets as absolute pointers, leading to bogus memory reads.
> 
> These flags were also never ported to execlist mode, making them no-ops
> on Gen9+ (which requires execlists), and Gen8 in the default mode.
> 
> On Gen8+, userspace can write these registers directly, achieving the
> same effect.  On Gen6-7.5, it likely makes sense to extend the command
> parser to support them.  I don't think anyone wants this on Gen4-5.
> 
> Based on a patch by Dave Gordon.
> 
> v3: Return -ENODEV for the getparam, as this is what we do for other
>     obsolete features.  Suggested by Chris Wilson.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92448
> Signed-off-by: Kenneth Graunke <kenneth@xxxxxxxxxxxxx>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx> [v2]
> Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> [v2]
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

Just give Daniel a chance to ack the ABI revert if he cares...
-Chris

> ---
>  drivers/gpu/drm/i915/i915_drv.c            |  4 +--
>  drivers/gpu/drm/i915/i915_drv.h            |  2 --
>  drivers/gpu/drm/i915/i915_gem.c            |  2 --
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c | 50 ++----------------------------
>  4 files changed, 3 insertions(+), 55 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
> index 309c29c84c54..448768cef1f6 100644
> --- a/drivers/gpu/drm/i915/i915_drv.c
> +++ b/drivers/gpu/drm/i915/i915_drv.c
> @@ -249,6 +249,7 @@ static int i915_getparam(struct drm_device *dev, void *data,
>  	case I915_PARAM_IRQ_ACTIVE:
>  	case I915_PARAM_ALLOW_BATCHBUFFER:
>  	case I915_PARAM_LAST_DISPATCH:
> +	case I915_PARAM_HAS_EXEC_CONSTANTS:
>  		/* Reject all old ums/dri params. */
>  		return -ENODEV;
>  	case I915_PARAM_CHIPSET_ID:
> @@ -275,9 +276,6 @@ static int i915_getparam(struct drm_device *dev, void *data,
>  	case I915_PARAM_HAS_BSD2:
>  		value = !!dev_priv->engine[VCS2];
>  		break;
> -	case I915_PARAM_HAS_EXEC_CONSTANTS:
> -		value = INTEL_GEN(dev_priv) >= 4;
> -		break;
>  	case I915_PARAM_HAS_LLC:
>  		value = HAS_LLC(dev_priv);
>  		break;
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 918118a268b8..8a6ba63d3f74 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2077,8 +2077,6 @@ struct drm_i915_private {
>  
>  	const struct intel_device_info info;
>  
> -	int relative_constants_mode;
> -
>  	void __iomem *regs;
>  
>  	struct intel_uncore uncore;
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 71297920fdf4..c9be0285c7cf 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -4656,8 +4656,6 @@ i915_gem_load_init(struct drm_i915_private *dev_priv)
>  	init_waitqueue_head(&dev_priv->gpu_error.wait_queue);
>  	init_waitqueue_head(&dev_priv->gpu_error.reset_queue);
>  
> -	dev_priv->relative_constants_mode = I915_EXEC_CONSTANTS_REL_GENERAL;
> -
>  	init_waitqueue_head(&dev_priv->pending_flip_queue);
>  
>  	dev_priv->mm.interruptible = true;
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index da0846fe2ad6..febb067903e9 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1412,10 +1412,7 @@ execbuf_submit(struct i915_execbuffer_params *params,
>  	       struct drm_i915_gem_execbuffer2 *args,
>  	       struct list_head *vmas)
>  {
> -	struct drm_i915_private *dev_priv = params->request->i915;
>  	u64 exec_start, exec_len;
> -	int instp_mode;
> -	u32 instp_mask, *cs;
>  	int ret;
>  
>  	ret = i915_gem_execbuffer_move_to_gpu(params->request, vmas);
> @@ -1426,54 +1423,11 @@ execbuf_submit(struct i915_execbuffer_params *params,
>  	if (ret)
>  		return ret;
>  
> -	instp_mode = args->flags & I915_EXEC_CONSTANTS_MASK;
> -	instp_mask = I915_EXEC_CONSTANTS_MASK;
> -	switch (instp_mode) {
> -	case I915_EXEC_CONSTANTS_REL_GENERAL:
> -	case I915_EXEC_CONSTANTS_ABSOLUTE:
> -	case I915_EXEC_CONSTANTS_REL_SURFACE:
> -		if (instp_mode != 0 && params->engine->id != RCS) {
> -			DRM_DEBUG("non-0 rel constants mode on non-RCS\n");
> -			return -EINVAL;
> -		}
> -
> -		if (instp_mode != dev_priv->relative_constants_mode) {
> -			if (INTEL_INFO(dev_priv)->gen < 4) {
> -				DRM_DEBUG("no rel constants on pre-gen4\n");
> -				return -EINVAL;
> -			}
> -
> -			if (INTEL_INFO(dev_priv)->gen > 5 &&
> -			    instp_mode == I915_EXEC_CONSTANTS_REL_SURFACE) {
> -				DRM_DEBUG("rel surface constants mode invalid on gen5+\n");
> -				return -EINVAL;
> -			}
> -
> -			/* The HW changed the meaning on this bit on gen6 */
> -			if (INTEL_INFO(dev_priv)->gen >= 6)
> -				instp_mask &= ~I915_EXEC_CONSTANTS_REL_SURFACE;
> -		}
> -		break;
> -	default:
> -		DRM_DEBUG("execbuf with unknown constants: %d\n", instp_mode);
> +	if (args->flags & I915_EXEC_CONSTANTS_MASK) {
> +		DRM_DEBUG("I915_EXEC_CONSTANTS_* unsupported\n");
>  		return -EINVAL;
>  	}
>  
> -	if (params->engine->id == RCS &&
> -	    instp_mode != dev_priv->relative_constants_mode) {
> -		cs = intel_ring_begin(params->request, 4);
> -		if (IS_ERR(cs))
> -			return PTR_ERR(cs);
> -
> -		*cs++ = MI_NOOP;
> -		*cs++ = MI_LOAD_REGISTER_IMM(1);
> -		*cs++ = i915_mmio_reg_offset(INSTPM);
> -		*cs++ = instp_mask << 16 | instp_mode;
> -		intel_ring_advance(params->request, cs);
> -
> -		dev_priv->relative_constants_mode = instp_mode;
> -	}
> -
>  	if (args->flags & I915_EXEC_GEN7_SOL_RESET) {
>  		ret = i915_reset_gen7_sol_offsets(params->request);
>  		if (ret)
> -- 
> 2.11.1
> 

-- 
Chris Wilson, Intel Open Source Technology Centre
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