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Subject: Re: [PATCH 1/2] coresight: tmc-etr: Correct memory sync ranges in SG
 mode
To:     Leo Yan <leo.yan@linaro.org>,
        Mathieu Poirier <mathieu.poirier@linaro.org>,
        Mike Leach <mike.leach@linaro.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
References: <20210710070206.462875-1-leo.yan@linaro.org>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
Message-ID: <3038eb59-155c-5c3d-18e3-e45e56b52523@arm.com>
Date:   Mon, 12 Jul 2021 11:25:54 +0100
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Hi Leo,

On 10/07/2021 08:02, Leo Yan wrote:
> Current code syncs the buffer range is [offset, offset+len), it doesn't
> consider the case when the trace data is wrapped around, in this case
> 'offset+len' is bigger than 'etr_buf->size'.  Thus it syncs buffer out
> of the memory buffer, and it also misses to sync buffer from the start
> of the memory.
> 

I doubt this claim is valid. We do the sync properly, taking the page
corresponding to the "offset" wrapping it around in "page" index.

Here is the code :


void tmc_sg_table_sync_data_range(struct tmc_sg_table *table,
                                   u64 offset, u64 size)
{
         int i, index, start;
         int npages = DIV_ROUND_UP(size, PAGE_SIZE);
         struct device *real_dev = table->dev->parent;
         struct tmc_pages *data = &table->data_pages;

         start = offset >> PAGE_SHIFT;
         for (i = start; i < (start + npages); i++) {
                 index = i % data->nr_pages;
                 dma_sync_single_for_cpu(real_dev, data->daddrs[index],
                                         PAGE_SIZE, DMA_FROM_DEVICE);
         }
}


See that the npages accounts for the "size" requested and we wrap the
"index" by the total number of pages in the buffer and pick the right
page.

So, I think this fix is not needed.

Cheers
Suzuki


> This patch corrects the memory sync ranges, when detects the wrapping
> around case, it splits into two chunks: one chunk is the tail of the
> buffer and another chunk is from the start of the buffer after wrapping
> around.
> 


> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
>   .../hwtracing/coresight/coresight-tmc-etr.c    | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index 888b0f929d33..a1afefcbf175 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -780,7 +780,23 @@ static void tmc_etr_sync_sg_buf(struct etr_buf *etr_buf, u64 rrp, u64 rwp)
>   	else
>   		etr_buf->len = ((w_offset < r_offset) ? etr_buf->size : 0) +
>   				w_offset - r_offset;
> -	tmc_sg_table_sync_data_range(table, r_offset, etr_buf->len);
> +
> +	if (r_offset + etr_buf->len > etr_buf->size) {
> +		int len1, len2;
> +
> +		/*
> +		 * If trace data is wrapped around, sync AUX bounce buffer
> +		 * for two chunks: "len1" is for the trace date length at
> +		 * the tail of bounce buffer, and "len2" is the length from
> +		 * the start of the buffer after wrapping around.
> +		 */
> +		len1 = etr_buf->size - r_offset;
> +		len2 = etr_buf->len - len1;
> +		tmc_sg_table_sync_data_range(table, r_offset, len1);
> +		tmc_sg_table_sync_data_range(table, 0, len2);
> +	} else {
> +		tmc_sg_table_sync_data_range(table, r_offset, etr_buf->len);
> +	}
>   }
>   
>   static const struct etr_buf_operations etr_sg_buf_ops = {
>