Re: [PATCH] drm/etnaviv: slow down FE idle polling

[Christian Gmeiner <christian.gmeiner@gmail.com>]

Hi Lucas

>
> Currently the FE is spinning way too fast when polling for new work in
> the FE idleloop. 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 so
> we may over/undershoot the target a bit, but that should be harmless.
>
> Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

Reviewed-by: Christian Gmeiner <cgmeiner@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);
>  }
>
>  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;
>  };
> --
> 2.39.2
>


-- 
greets
--
Christian Gmeiner, MSc

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