[PATCH v4] coresight: tmc-etr: Speed up for bounce buffer in flat mode

[Mathieu Poirier]

On Sun, Sep 05, 2021 at 11:21:44AM +0800, Leo Yan wrote:
> The AUX bounce buffer is allocated with API dma_alloc_coherent(), in the
> low level's architecture code, e.g. for Arm64, it maps the memory with
> the attribution "Normal non-cacheable"; this can be concluded from the
> definition for pgprot_dmacoherent() in arch/arm64/include/asm/pgtable.h.
> 
> Later when access the AUX bounce buffer, since the memory mapping is
> non-cacheable, it's low efficiency due to every load instruction must
> reach out DRAM.
> 
> This patch changes to allocate pages with dma_alloc_noncoherent(), the
> driver can access the memory via cacheable mapping; therefore, load
> instructions can fetch data from cache lines rather than always read
> data from DRAM, the driver can boost memory performance.  After using
> the cacheable mapping, the driver uses dma_sync_single_for_cpu() to
> invalidate cacheline prior to read bounce buffer so can avoid read stale
> trace data.
> 
> By measurement the duration for function tmc_update_etr_buffer() with
> ftrace function_graph tracer, it shows the performance significant
> improvement for copying 4MiB data from bounce buffer:
> 
>   # echo tmc_etr_get_data_flat_buf > set_graph_notrace // avoid noise
>   # echo tmc_update_etr_buffer > set_graph_function
>   # echo function_graph > current_tracer
> 
>   before:
> 
>   # CPU  DURATION                  FUNCTION CALLS
>   # |     |   |                     |   |   |   |
>   2)               |    tmc_update_etr_buffer() {
>   ...
>   2) # 8148.320 us |    }
> 
>   after:
> 
>   # CPU  DURATION                  FUNCTION CALLS
>   # |     |   |                     |   |   |   |
>   2)               |  tmc_update_etr_buffer() {
>   ...
>   2) # 2525.420 us |  }
> 
> Signed-off-by: Leo Yan <leo.yan at linaro.org>
> Reviewed-by: Suzuki K Poulose <suzuki.poulose at arm.com>

I have merged this patch.

Thanks,
Mathieu

> ---
> 
> Changes from v3:
> Refined change to use dma_alloc_noncoherent()/dma_free_noncoherent()
> (Robin Murphy);
> Retested functionality and performance on Juno-r2 board.
> 
> Changes from v2:
> Sync the entire buffer in one go when the tracing is wrap around
> (Suzuki);
> Add Suzuki's review tage.
> 
>  .../hwtracing/coresight/coresight-tmc-etr.c   | 26 ++++++++++++++++---
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index acdb59e0e661..a049b525a274 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -609,8 +609,9 @@ static int tmc_etr_alloc_flat_buf(struct tmc_drvdata *drvdata,
>  	if (!flat_buf)
>  		return -ENOMEM;
>  
> -	flat_buf->vaddr = dma_alloc_coherent(real_dev, etr_buf->size,
> -					     &flat_buf->daddr, GFP_KERNEL);
> +	flat_buf->vaddr = dma_alloc_noncoherent(real_dev, etr_buf->size,
> +						&flat_buf->daddr,
> +						DMA_FROM_DEVICE, GFP_KERNEL);
>  	if (!flat_buf->vaddr) {
>  		kfree(flat_buf);
>  		return -ENOMEM;
> @@ -631,14 +632,18 @@ static void tmc_etr_free_flat_buf(struct etr_buf *etr_buf)
>  	if (flat_buf && flat_buf->daddr) {
>  		struct device *real_dev = flat_buf->dev->parent;
>  
> -		dma_free_coherent(real_dev, flat_buf->size,
> -				  flat_buf->vaddr, flat_buf->daddr);
> +		dma_free_noncoherent(real_dev, etr_buf->size,
> +				     flat_buf->vaddr, flat_buf->daddr,
> +				     DMA_FROM_DEVICE);
>  	}
>  	kfree(flat_buf);
>  }
>  
>  static void tmc_etr_sync_flat_buf(struct etr_buf *etr_buf, u64 rrp, u64 rwp)
>  {
> +	struct etr_flat_buf *flat_buf = etr_buf->private;
> +	struct device *real_dev = flat_buf->dev->parent;
> +
>  	/*
>  	 * Adjust the buffer to point to the beginning of the trace data
>  	 * and update the available trace data.
> @@ -648,6 +653,19 @@ static void tmc_etr_sync_flat_buf(struct etr_buf *etr_buf, u64 rrp, u64 rwp)
>  		etr_buf->len = etr_buf->size;
>  	else
>  		etr_buf->len = rwp - rrp;
> +
> +	/*
> +	 * The driver always starts tracing at the beginning of the buffer,
> +	 * the only reason why we would get a wrap around is when the buffer
> +	 * is full.  Sync the entire buffer in one go for this case.
> +	 */
> +	if (etr_buf->offset + etr_buf->len > etr_buf->size)
> +		dma_sync_single_for_cpu(real_dev, flat_buf->daddr,
> +					etr_buf->size, DMA_FROM_DEVICE);
> +	else
> +		dma_sync_single_for_cpu(real_dev,
> +					flat_buf->daddr + etr_buf->offset,
> +					etr_buf->len, DMA_FROM_DEVICE);
>  }
>  
>  static ssize_t tmc_etr_get_data_flat_buf(struct etr_buf *etr_buf,
> -- 
> 2.25.1
>