Re: [PATCH] dmaengine: stm32-dma: fix residue calculation in stm32-dma

[Vinod Koul]

Hi Arnaud,

Sorry for delay in review, the conference travel/vacation plan delayed
this.

On 27-03-19, 13:21, Arnaud Pouliquen wrote:
> During residue calculation. the DMA can switch to the next sg. When
> this race condition occurs, the residue returned value is not valid.
> Indeed the position in the sg returned by the hardware is the position
> of the next sg, not the current sg.
> Solution is to check the sg after the calculation to verify it.
> If a transition is detected we consider that the DMA has switched to
> the beginning of next sg.

Now, that sounds like duct tape. Why should we bother doing that.

Also looking back at the stm32_dma_desc_residue() and calls to it from
stm32_dma_tx_status() am not sure we are doing the right thing

why are we looking at next_sg here, can you explain me that please

> 
> Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@xxxxxx>
> Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@xxxxxx>
> ---
>  drivers/dma/stm32-dma.c | 70 ++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 57 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/dma/stm32-dma.c b/drivers/dma/stm32-dma.c
> index 4903a40..30309d2 100644
> --- a/drivers/dma/stm32-dma.c
> +++ b/drivers/dma/stm32-dma.c
> @@ -1038,33 +1038,77 @@ static u32 stm32_dma_get_remaining_bytes(struct stm32_dma_chan *chan)
>  	return ndtr << width;
>  }
>  
> +static bool stm32_dma_is_current_sg(struct stm32_dma_chan *chan)
> +{
> +	struct stm32_dma_device *dmadev = stm32_dma_get_dev(chan);
> +	struct stm32_dma_sg_req *sg_req;
> +	u32 dma_scr, dma_smar, id;
> +
> +	id = chan->id;
> +	dma_scr = stm32_dma_read(dmadev, STM32_DMA_SCR(id));
> +
> +	if (!(dma_scr & STM32_DMA_SCR_DBM))
> +		return true;
> +
> +	sg_req = &chan->desc->sg_req[chan->next_sg];
> +
> +	if (dma_scr & STM32_DMA_SCR_CT) {
> +		dma_smar = stm32_dma_read(dmadev, STM32_DMA_SM0AR(id));
> +		return (dma_smar == sg_req->chan_reg.dma_sm0ar);
> +	}
> +
> +	dma_smar = stm32_dma_read(dmadev, STM32_DMA_SM1AR(id));
> +
> +	return (dma_smar == sg_req->chan_reg.dma_sm1ar);
> +}
> +
>  static size_t stm32_dma_desc_residue(struct stm32_dma_chan *chan,
>  				     struct stm32_dma_desc *desc,
>  				     u32 next_sg)
>  {
>  	u32 modulo, burst_size;
> -	u32 residue = 0;
> +	u32 residue;
> +	u32 n_sg = next_sg;
> +	struct stm32_dma_sg_req *sg_req = &chan->desc->sg_req[chan->next_sg];
>  	int i;
>  
> +	residue = stm32_dma_get_remaining_bytes(chan);
> +
>  	/*
> -	 * In cyclic mode, for the last period, residue = remaining bytes from
> -	 * NDTR
> +	 * Calculate the residue means compute the descriptors
> +	 * information:
> +	 * - the sg currently transferred
> +	 * - the remaining position in this sg (NDTR).
> +	 *
> +	 * The issue is that a race condition can occur if DMA is
> +	 * running. DMA can have started to transfer the next sg before
> +	 * the position in sg is read. In this case the remaing position
> +	 * can correspond to the new sg position.
> +	 * The strategy implemented in the stm32 driver is to check the
> +	 * sg transition. If detected we can not trust the SxNDTR register
> +	 * value, this register can not be up to date during the transition.
> +	 * In this case we can assume that the dma is at the beginning of next
> +	 * sg so we calculate the residue in consequence.
>  	 */
> -	if (chan->desc->cyclic && next_sg == 0) {
> -		residue = stm32_dma_get_remaining_bytes(chan);
> -		goto end;
> +
> +	if (!stm32_dma_is_current_sg(chan)) {
> +		n_sg++;
> +		if (n_sg == chan->desc->num_sgs)
> +			n_sg = 0;
> +		residue = sg_req->len;
>  	}
>  
>  	/*
> -	 * For all other periods in cyclic mode, and in sg mode,
> -	 * residue = remaining bytes from NDTR + remaining periods/sg to be
> -	 * transferred
> +	 * In cyclic mode, for the last period, residue = remaining bytes
> +	 * from NDTR,
> +	 * else for all other periods in cyclic mode, and in sg mode,
> +	 * residue = remaining bytes from NDTR + remaining
> +	 * periods/sg to be transferred
>  	 */
> -	for (i = next_sg; i < desc->num_sgs; i++)
> -		residue += desc->sg_req[i].len;
> -	residue += stm32_dma_get_remaining_bytes(chan);
> +	if (!chan->desc->cyclic || n_sg != 0)
> +		for (i = n_sg; i < desc->num_sgs; i++)
> +			residue += desc->sg_req[i].len;
>  
> -end:
>  	if (!chan->mem_burst)
>  		return residue;
>  
> -- 
> 2.7.4

-- 
~Vinod