Re: [PATCH] dmaengine: bcm2835: Fix polling for completion of DMA with interrupts masked.

[Vinod Koul <vinod.koul@intel.com>]

On Fri, Jun 03, 2016 at 07:29:11PM -0700, Eric Anholt wrote:
> The tx_status hook is supposed to be safe to call from interrupt
> context, but it wouldn't ever return completion for the last transfer,
> meaning you couldn't poll for DMA completion with interrupts masked.

and why is that?

> This fixes IRQ handling for bcm2835's DSI1, which requires using the
> DMA engine to write its registers due to a bug in the AXI bridge.
> 
> Signed-off-by: Eric Anholt <eric@anholt.net>
> ---
>  drivers/dma/bcm2835-dma.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
> index 6149b27c33ad..320461c578e3 100644
> --- a/drivers/dma/bcm2835-dma.c
> +++ b/drivers/dma/bcm2835-dma.c
> @@ -570,16 +570,16 @@ static enum dma_status bcm2835_dma_tx_status(struct dma_chan *chan,
>  	struct virt_dma_desc *vd;
>  	enum dma_status ret;
>  	unsigned long flags;
> +	u32 residue;
>  
>  	ret = dma_cookie_status(chan, cookie, txstate);
> -	if (ret == DMA_COMPLETE || !txstate)
> +	if (ret == DMA_COMPLETE)

Why do you change this? txstate can be NULL, so no point calculating reside
for those cases

>  		return ret;
>  
>  	spin_lock_irqsave(&c->vc.lock, flags);
>  	vd = vchan_find_desc(&c->vc, cookie);
>  	if (vd) {
> -		txstate->residue =
> -			bcm2835_dma_desc_size(to_bcm2835_dma_desc(&vd->tx));
> +		residue = bcm2835_dma_desc_size(to_bcm2835_dma_desc(&vd->tx));
>  	} else if (c->desc && c->desc->vd.tx.cookie == cookie) {
>  		struct bcm2835_desc *d = c->desc;
>  		dma_addr_t pos;
> @@ -591,11 +591,25 @@ static enum dma_status bcm2835_dma_tx_status(struct dma_chan *chan,
>  		else
>  			pos = 0;
>  
> -		txstate->residue = bcm2835_dma_desc_size_pos(d, pos);
> +		residue = bcm2835_dma_desc_size_pos(d, pos);
> +
> +		/*
> +		 * If our non-cyclic transfer is done, then report
> +		 * complete and trigger the next tx now.  This lets
> +		 * the dmaengine API be used synchronously from an IRQ
> +		 * handler.
> +		 */
> +		if (!d->cyclic && residue == 0) {
> +			vchan_cookie_complete(&c->desc->vd);
> +			bcm2835_dma_start_desc(c);
> +			ret = dma_cookie_status(chan, cookie, txstate);
> +		}
>  	} else {
> -		txstate->residue = 0;
> +		residue = 0;
>  	}
>  
> +	dma_set_residue(txstate, residue);
> +
>  	spin_unlock_irqrestore(&c->vc.lock, flags);
>  
>  	return ret;
> -- 
> 2.8.0.rc3
> 

-- 
~Vinod