[Mesa-dev] [PATCH 2/2] i965: Use absolute addressing for constant buffer 0 on Kernel 4.16+.

[Francisco Jerez]

Kenneth Graunke <kenneth at whitecape.org> writes:

> By default, 3DSTATE_CONSTANT_* Constant Buffer 0 is relative to dynamic
> state base address.  This makes it unusable for pushing UBOs.
>
> There is a bit in the INSTPM register (or CS_DEBUG_MODE2 on Skylake)
> which controls whether buffer 0 is relative to dynamic state base
> address, or simply a normal pointer.  Setting that gives us full
> flexibility.  This lets us push up to 4 UBO ranges.
>
> We can't currently write this on Haswell and earlier, and will need
> to update the kernel command parser, and then do the whole version
> checking song and dance.  We also need a brand new kernel that supports
> context isolation - on older kernels, newly created contexts inherit
> register state from whatever happened to be running.  So, setting this
> would have catastrophic impact on other drivers such as libva, Beignet,
> or older Mesa.
>
> See commit 8ec5a4e4a4a32f4de351c5fc2bf0eb615b6eef1b where we did this
> once before, but had to revert it in commit 013d33122028f2492da90a03a.

Reviewed-by: Francisco Jerez <currojerez at riseup.net>

> ---
>  src/mesa/drivers/dri/i965/brw_state_upload.c | 24 ++++++++++++++++++++++++
>  src/mesa/drivers/dri/i965/intel_screen.c     |  9 ++++++++-
>  2 files changed, 32 insertions(+), 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_state_upload.c b/src/mesa/drivers/dri/i965/brw_state_upload.c
> index 86c12e4d357..d8273aa5734 100644
> --- a/src/mesa/drivers/dri/i965/brw_state_upload.c
> +++ b/src/mesa/drivers/dri/i965/brw_state_upload.c
> @@ -49,6 +49,7 @@ static void
>  brw_upload_initial_gpu_state(struct brw_context *brw)
>  {
>     const struct gen_device_info *devinfo = &brw->screen->devinfo;
> +   const struct brw_compiler *compiler = brw->screen->compiler;
>  
>     /* On platforms with hardware contexts, we can set our initial GPU state
>      * right away rather than doing it via state atoms.  This saves a small
> @@ -115,6 +116,29 @@ brw_upload_initial_gpu_state(struct brw_context *brw)
>        OUT_BATCH(0);
>        ADVANCE_BATCH();
>     }
> +
> +   /* Set the "CONSTANT_BUFFER Address Offset Disable" bit, so
> +    * 3DSTATE_CONSTANT_XS buffer 0 is an absolute address.
> +    *
> +    * This is only safe on kernels with context isolation support.
> +    */
> +   if (!compiler->constant_buffer_0_is_relative) {
> +      if (devinfo->gen >= 9) {
> +         BEGIN_BATCH(3);
> +         OUT_BATCH(MI_LOAD_REGISTER_IMM | (3 - 2));
> +         OUT_BATCH(CS_DEBUG_MODE2);
> +         OUT_BATCH(REG_MASK(CSDBG2_CONSTANT_BUFFER_ADDRESS_OFFSET_DISABLE) |
> +                   CSDBG2_CONSTANT_BUFFER_ADDRESS_OFFSET_DISABLE);
> +         ADVANCE_BATCH();
> +      } else if (devinfo->gen == 8) {
> +         BEGIN_BATCH(3);
> +         OUT_BATCH(MI_LOAD_REGISTER_IMM | (3 - 2));
> +         OUT_BATCH(INSTPM);
> +         OUT_BATCH(REG_MASK(INSTPM_CONSTANT_BUFFER_ADDRESS_OFFSET_DISABLE) |
> +                   INSTPM_CONSTANT_BUFFER_ADDRESS_OFFSET_DISABLE);
> +         ADVANCE_BATCH();
> +      }
> +   }
>  }
>  
>  static inline const struct brw_tracked_state *
> diff --git a/src/mesa/drivers/dri/i965/intel_screen.c b/src/mesa/drivers/dri/i965/intel_screen.c
> index d275b28a4b9..4b62c5f66cb 100644
> --- a/src/mesa/drivers/dri/i965/intel_screen.c
> +++ b/src/mesa/drivers/dri/i965/intel_screen.c
> @@ -2694,7 +2694,14 @@ __DRIconfig **intelInitScreen2(__DRIscreen *dri_screen)
>     screen->compiler = brw_compiler_create(screen, devinfo);
>     screen->compiler->shader_debug_log = shader_debug_log_mesa;
>     screen->compiler->shader_perf_log = shader_perf_log_mesa;
> -   screen->compiler->constant_buffer_0_is_relative = true;
> +
> +   /* Changing the meaning of constant buffer pointers from a dynamic state
> +    * offset to an absolute address is only safe if the kernel isolates other
> +    * contexts from our changes.
> +    */
> +   screen->compiler->constant_buffer_0_is_relative = devinfo->gen < 8 ||
> +      !(screen->kernel_features & KERNEL_ALLOWS_CONTEXT_ISOLATION);
> +
>     screen->compiler->supports_pull_constants = true;
>  
>     screen->has_exec_fence =
> -- 
> 2.16.1
>
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> mesa-dev at lists.freedesktop.org
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