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

[Pierre Yves MORDRET]

Hello

Reviewed-by: Pierre-Yves MORDRET <pierre-yves.mordret@xxxxxx>

Thanks

On 4/23/19 5:18 PM, Arnaud Pouliquen wrote:
> Hello Vinod,
> 
> Just a gentle reminder, if you could take a moment to review this patch.
> FYI, the patch has already been internally reviewed by Pierre Yves
> mordret...
> His sign-off is in the commit message.
> 
> Thanks,
> 
> Arnaud
> 
> 
> On 3/27/19 1:21 PM, 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.
>>
>> 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;
>>  
>>