Re: [PATCH v2] drm/xe/xe2: Enable Priority Mem Read

["Souza, Jose" <jose.souza@intel.com>]

On Thu, 2024-05-30 at 11:55 +0530, Pallavi Mishra wrote:
> Enable feature to allow memory reads to take a
> priority memory path. This will reduce latency
> on the read path, but may introduce read after
> write (RAW) hazards as read and writes will no
> longer be ordered.
> 
> To avoid RAW hazards, SW can use the MI_MEM_FENCE
> command or any other MI command that generates
> non posted memory writes. This will ensure data
> is coherent in memory prior to execution of
> commands which read data from memory.

This has the potential to cause issues in UMDs, Mesa for example don't have any MI_MEM_FENCE instruction in the code base.

So please before merging this sync with all UMDs or merge it behind a run-time or build time parameter with it disabled by default.

> 
> No pattern identified in KMD that could lead to a
> hazard.
> 
> v2: Modify commit message, enable priority mem
> read feature for media, modify version range,
> modify bspec detail (Matt Roper)
> 
> Bspec: 60298, 60237, 60187, 60188
> 
> Signed-off-by: Pallavi Mishra <pallavi.mishra@intel.com>
> ---
>  drivers/gpu/drm/xe/regs/xe_engine_regs.h |  1 +
>  drivers/gpu/drm/xe/xe_hw_engine.c        | 11 +++++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/regs/xe_engine_regs.h b/drivers/gpu/drm/xe/regs/xe_engine_regs.h
> index 263ffc7bc2ef..4e8f9a61f0bf 100644
> --- a/drivers/gpu/drm/xe/regs/xe_engine_regs.h
> +++ b/drivers/gpu/drm/xe/regs/xe_engine_regs.h
> @@ -104,6 +104,7 @@
>  #define CSFE_CHICKEN1(base)			XE_REG((base) + 0xd4, XE_REG_OPTION_MASKED)
>  #define   GHWSP_CSB_REPORT_DIS			REG_BIT(15)
>  #define   PPHWSP_CSB_AND_TIMESTAMP_REPORT_DIS	REG_BIT(14)
> +#define   CS_PRIORITY_MEM_READ			REG_BIT(7)
>  
>  #define FF_SLICE_CS_CHICKEN1(base)		XE_REG((base) + 0xe0, XE_REG_OPTION_MASKED)
>  #define   FFSC_PERCTX_PREEMPT_CTRL		REG_BIT(14)
> diff --git a/drivers/gpu/drm/xe/xe_hw_engine.c b/drivers/gpu/drm/xe/xe_hw_engine.c
> index 9eef789be897..f03e5cc9063e 100644
> --- a/drivers/gpu/drm/xe/xe_hw_engine.c
> +++ b/drivers/gpu/drm/xe/xe_hw_engine.c
> @@ -424,6 +424,17 @@ hw_engine_setup_default_state(struct xe_hw_engine *hwe)
>  					   0xA,
>  					   XE_RTP_ACTION_FLAG(ENGINE_BASE)))
>  		},
> +		/* Enable Priority Mem Read */
> +		{ XE_RTP_NAME("Priority_Mem_Read"),
> +		  XE_RTP_RULES(GRAPHICS_VERSION_RANGE(2001, XE_RTP_END_VERSION_UNDEFINED)),
> +		  XE_RTP_ACTIONS(SET(CSFE_CHICKEN1(0), CS_PRIORITY_MEM_READ,
> +				     XE_RTP_ACTION_FLAG(ENGINE_BASE)))
> +		},
> +		{ XE_RTP_NAME("Priority_Mem_Read_For_Media"),
> +		  XE_RTP_RULES(MEDIA_VERSION_RANGE(1301, XE_RTP_END_VERSION_UNDEFINED)),
> +		  XE_RTP_ACTIONS(SET(CSFE_CHICKEN1(0), CS_PRIORITY_MEM_READ,
> +				     XE_RTP_ACTION_FLAG(ENGINE_BASE)))
> +		},
>  		{}
>  	};
>