drm/etnaviv: slow down FE idle polling

[Sui Jingfeng]

Hi,

On 2023/6/7 20:59, Lucas Stach wrote:
> Currently the FE is spinning way too fast when polling for new work in
'way' -> 'away'
> the FE idleloop.
'idleloop' -> 'idle loop'
>   As each poll fetches 16 bytes from memory, a GPU running
> at 1GHz with the current setting of 200 wait cycle between fetches causes
> 80 MB/s of memory traffic just to check for new work when the GPU is
> otherwise idle, which is more FE traffic than in some GPU loaded cases.
>
> Significantly increase the number of wait cycles to slow down the poll
> interval to ~30µs, limiting the FE idle memory traffic to 512 KB/s, while
> providing a max latency which should not hurt most use-cases. The FE WAIT
> command seems to have some unknown discrete steps in the wait cycles
add a comma here.
>   so
> we may over/undershoot the target a bit, but that should be harmless.
overshoot or undershoot
> Signed-off-by: Lucas Stach <l.stach at pengutronix.de>
> Reviewed-by: Christian Gmeiner <cgmeiner at igalia.com>
> ---
>   drivers/gpu/drm/etnaviv/etnaviv_buffer.c | 11 ++++++-----
>   drivers/gpu/drm/etnaviv/etnaviv_gpu.c    |  7 +++++++
>   drivers/gpu/drm/etnaviv/etnaviv_gpu.h    |  1 +
>   3 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_buffer.c b/drivers/gpu/drm/etnaviv/etnaviv_buffer.c
> index cf741c5c82d2..384df1659be6 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_buffer.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_buffer.c
> @@ -53,11 +53,12 @@ static inline void CMD_END(struct etnaviv_cmdbuf *buffer)
>   	OUT(buffer, VIV_FE_END_HEADER_OP_END);
>   }
>   
> -static inline void CMD_WAIT(struct etnaviv_cmdbuf *buffer)
> +static inline void CMD_WAIT(struct etnaviv_cmdbuf *buffer,
> +			    unsigned int waitcycles)
>   {
>   	buffer->user_size = ALIGN(buffer->user_size, 8);
>   
> -	OUT(buffer, VIV_FE_WAIT_HEADER_OP_WAIT | 200);
> +	OUT(buffer, VIV_FE_WAIT_HEADER_OP_WAIT | waitcycles);
>   }
>   
>   static inline void CMD_LINK(struct etnaviv_cmdbuf *buffer,
> @@ -168,7 +169,7 @@ u16 etnaviv_buffer_init(struct etnaviv_gpu *gpu)
>   	/* initialize buffer */
>   	buffer->user_size = 0;
>   
> -	CMD_WAIT(buffer);
> +	CMD_WAIT(buffer, gpu->fe_waitcycles);
>   	CMD_LINK(buffer, 2,
>   		 etnaviv_cmdbuf_get_va(buffer, &gpu->mmu_context->cmdbuf_mapping)
>   		 + buffer->user_size - 4);
> @@ -320,7 +321,7 @@ void etnaviv_sync_point_queue(struct etnaviv_gpu *gpu, unsigned int event)
>   	CMD_END(buffer);
>   
>   	/* Append waitlink */
> -	CMD_WAIT(buffer);
> +	CMD_WAIT(buffer, gpu->fe_waitcycles);
>   	CMD_LINK(buffer, 2,
>   		 etnaviv_cmdbuf_get_va(buffer, &gpu->mmu_context->cmdbuf_mapping)
>   		 + buffer->user_size - 4);
> @@ -503,7 +504,7 @@ void etnaviv_buffer_queue(struct etnaviv_gpu *gpu, u32 exec_state,
>   
>   	CMD_LOAD_STATE(buffer, VIVS_GL_EVENT, VIVS_GL_EVENT_EVENT_ID(event) |
>   		       VIVS_GL_EVENT_FROM_PE);
> -	CMD_WAIT(buffer);
> +	CMD_WAIT(buffer, gpu->fe_waitcycles);
>   	CMD_LINK(buffer, 2,
>   		 etnaviv_cmdbuf_get_va(buffer, &gpu->mmu_context->cmdbuf_mapping)
>   		 + buffer->user_size - 4);
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gpu.c b/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
> index 41aab1aa330b..8c20dff32240 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gpu.c
> @@ -493,6 +493,13 @@ static void etnaviv_gpu_update_clock(struct etnaviv_gpu *gpu)
>   		clock |= VIVS_HI_CLOCK_CONTROL_FSCALE_VAL(fscale);
>   		etnaviv_gpu_load_clock(gpu, clock);
>   	}
> +
> +	/*
> +	 * Choose number of wait cycles to target a ~30us (1/32768) max latency
> +	 * until new work is picked up by the FE when it polls in the idle loop.
> +	 */
> +	gpu->fe_waitcycles = min(gpu->base_rate_core >> (15 - gpu->freq_scale),
> +				 0xffffUL);

This patch is NOT effective on our hardware GC1000 v5037 (ls7a1000 + 
ls3a5000).

As the gpu->base_rate_core is 0,  so, in the end gpu->fe_waitcycles is 
also zero.

But after apply this path, the glmark2 still run happily, no influence. So


Tested-by: Sui Jingfeng <suijingfeng at loongson.cn>

>   }
>   
>   static int etnaviv_hw_reset(struct etnaviv_gpu *gpu)
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gpu.h b/drivers/gpu/drm/etnaviv/etnaviv_gpu.h
> index 98c6f9c320fc..e1e1de59c38d 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gpu.h
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gpu.h
> @@ -150,6 +150,7 @@ struct etnaviv_gpu {
>   	struct clk *clk_shader;
>   
>   	unsigned int freq_scale;
> +	unsigned int fe_waitcycles;
>   	unsigned long base_rate_core;
>   	unsigned long base_rate_shader;
>   };

-- 
Jingfeng