Re: [PATCH 1/6] dma: rcar-dma: add wait after stopping dma engine

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

On Tue, Sep 29, 2015 at 10:44:43PM +0200, hamzahfrq.sub@gmail.com wrote:
> From: Muhammad Hamza Farooq <mfarooq@visteon.com>
> 
> DMA engine does not stop instantaneously when transaction is going on
> (see datasheet). Wait has been added

Hmmm, why is threading broken in this series?
Also why is this PATCH 1/6 whereas the rest of series is v3!

Right way is to let git do this for you and use git format-patch
--subject-prefix="PATCH v3"...

You don't do these things manually, that would be terrible waste of time

Also use git send-email to keep threading

> Signed-off-by: Muhammad Hamza Farooq <mfarooq@visteon.com>
> ---
>  drivers/dma/sh/rcar-dmac.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/dma/sh/rcar-dmac.c b/drivers/dma/sh/rcar-dmac.c
> index 7820d07..2b28291 100644
> --- a/drivers/dma/sh/rcar-dmac.c
> +++ b/drivers/dma/sh/rcar-dmac.c
> @@ -716,14 +716,38 @@ static int rcar_dmac_fill_hwdesc(struct rcar_dmac_chan *chan,
>  /* -----------------------------------------------------------------------------
>   * Stop and reset
>   */
> +#define NR_READS_TO_WAIT 5 /* number of times to check if DE = 0 */
> +static inline int rcar_dmac_wait_stop(struct rcar_dmac_chan *chan)
> +{
> +	unsigned int i = 0;
> +
> +	do {
> +		u32 chcr = rcar_dmac_chan_read(chan, RCAR_DMACHCR);
> +
> +		if (!(chcr & RCAR_DMACHCR_DE))
> +			return 0;
> +		cpu_relax();
> +		dev_dbg(chan->chan.device->dev, "DMA xfer couldn't be stopped");

And user has no way of knowing this on production system

> +	} while (++i < NR_READS_TO_WAIT);
>  
> -static void rcar_dmac_chan_halt(struct rcar_dmac_chan *chan)
> +	return -EBUSY;
> +}
> +
> +/* Called with chan lock held */
> +static int rcar_dmac_chan_halt(struct rcar_dmac_chan *chan)
>  {
> -	u32 chcr = rcar_dmac_chan_read(chan, RCAR_DMACHCR);
> +	u32 chcr;
> +	int ret;
>  
> +	chcr = rcar_dmac_chan_read(chan, RCAR_DMACHCR);
>  	chcr &= ~(RCAR_DMACHCR_DSE | RCAR_DMACHCR_DSIE | RCAR_DMACHCR_IE |
>  		  RCAR_DMACHCR_TE | RCAR_DMACHCR_DE);
>  	rcar_dmac_chan_write(chan, RCAR_DMACHCR, chcr);
> +	ret = rcar_dmac_wait_stop(chan);
> +
> +	WARN_ON(ret < 0);

What if we have broken board and will see dmesg storm for this, perhaps
WARN_ON_ONCE and also would be helpful to use the verbose version and print
some meaningful message

-- 
~Vinod