linux-kernel - Re: [PATCH v5 2/7] dmaengine: xilinx_dma: in axidma slave_sg and dma

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Date:   Tue, 18 Sep 2018 09:21:05 -0700
From:   Vinod <vkoul@...nel.org>
To:     Andrea Merello <andrea.merello@...il.com>
Cc:     dan.j.williams@...el.com, michal.simek@...inx.com,
        appana.durga.rao@...inx.com, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        robh+dt@...nel.org, mark.rutland@....com,
        devicetree@...r.kernel.org, radhey.shyam.pandey@...inx.com
Subject: Re: [PATCH v5 2/7] dmaengine: xilinx_dma: in axidma slave_sg and
 dma_cyclic mode align split descriptors

On 07-09-18, 08:24, Andrea Merello wrote:
> Whenever a single or cyclic transaction is prepared, the driver
> could eventually split it over several SG descriptors in order
> to deal with the HW maximum transfer length.
> 
> This could end up in DMA operations starting from a misaligned
> address. This seems fatal for the HW if DRE (Data Realignment Engine)
> is not enabled.
> 
> This patch eventually adjusts the transfer size in order to make sure
> all operations start from an aligned address.
> 
> Cc: Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
> Signed-off-by: Andrea Merello <andrea.merello@...il.com>
> Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
> ---
> Changes in v2:
>         - don't introduce copy_mask field, rather rely on already-esistent
>           copy_align field. Suggested by Radhey Shyam Pandey
>         - reword title
> Changes in v3:
> 	- fix bug introduced in v2: wrong copy size when DRE is enabled
> 	- use implementation suggested by Radhey Shyam Pandey
> Changes in v4:
> 	- rework on the top of 1/6
> Changes in v5:
> 	- fix typo in commit title
> 	- add hint about "DRE" meaning in commit message
> ---
>  drivers/dma/xilinx/xilinx_dma.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/dma/xilinx/xilinx_dma.c b/drivers/dma/xilinx/xilinx_dma.c
> index a3aaa0e34cc7..aaa6de8a70e4 100644
> --- a/drivers/dma/xilinx/xilinx_dma.c
> +++ b/drivers/dma/xilinx/xilinx_dma.c
> @@ -954,15 +954,28 @@ static int xilinx_dma_alloc_chan_resources(struct dma_chan *dchan)
>  
>  /**
>   * xilinx_dma_calc_copysize - Calculate the amount of data to copy
> + * @chan: Driver specific DMA channel
>   * @size: Total data that needs to be copied
>   * @done: Amount of data that has been already copied
>   *
>   * Return: Amount of data that has to be copied
>   */
> -static int xilinx_dma_calc_copysize(int size, int done)
> +static int xilinx_dma_calc_copysize(struct xilinx_dma_chan *chan,
> +				    int size, int done)

align to preceeding line opening brace please

>  {
> -	return min_t(size_t, size - done,
> +	size_t copy = min_t(size_t, size - done,
>  		     XILINX_DMA_MAX_TRANS_LEN);

so we can do this way in patch 1:

        size t copy;

        copy =  min_t(size_t, size - done,
                      XILINX_DMA_MAX_TRANS_LEN);

        return copy;

and then add these here, feels like we are redoing change introduced in
patch 1..


> +	if ((copy + done < size) &&
> +	    chan->xdev->common.copy_align) {
> +		/*
> +		 * If this is not the last descriptor, make sure
> +		 * the next one will be properly aligned
> +		 */
> +		copy = rounddown(copy,
> +				 (1 << chan->xdev->common.copy_align));
> +	}
> +	return copy;
>  }
>  
>  /**
> @@ -1804,7 +1817,7 @@ static struct dma_async_tx_descriptor *xilinx_dma_prep_slave_sg(
>  			 * Calculate the maximum number of bytes to transfer,
>  			 * making sure it is less than the hw limit
>  			 */
> -			copy = xilinx_dma_calc_copysize(sg_dma_len(sg),
> +			copy = xilinx_dma_calc_copysize(chan, sg_dma_len(sg),

why not keep chan in patch 1 and add only handling in patch 2, seems
less churn to me..

-- 
~Vinod