RE: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer

[Robin Gong]

> -----Original Message-----
> From: Lucas Stach [mailto:l.stach@xxxxxxxxxxxxxx]
> Sent: 2018å7æ23æ 18:54
> To: Robin Gong <yibin.gong@xxxxxxx>; vkoul@xxxxxxxxxx;
> dan.j.williams@xxxxxxxxx; s.hauer@xxxxxxxxxxxxxx; linux@xxxxxxxxxxxxxxx
> Cc: dmaengine@xxxxxxxxxxxxxxx; dl-linux-imx <linux-imx@xxxxxxx>;
> kernel@xxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds for one
> transfer
> 
> Am Dienstag, den 24.07.2018, 01:46 +0800 schrieb Robin Gong:
> > If multi-bds used in one transfer, all bds should be consisten
> > memory.To easily follow it, enlarge the dma pool size into 20 bds, and
> > it will report error if the number of bds is over than 20. For
> > dmatest, the max count for single transfer is NUM_BD *
> SDMA_BD_MAX_CNT
> > = 20 * 65535 = ~1.28MB.
> 
> Both the commit message and the comment need a lot more care to actually
> tell what this commit is trying to achieve. Currently I don't follow at all. What
> does "consisten" mean? Do you mean BDs should be contiguous in memory?
Yes, BDs should be contiguous  one by one in memory.
> 
> What do you gain by over-allocating each BD by a factor of 20?
I guess dma_pool_alloc will return error in such case, and then cause dma setup
transfer failure.
> 
> Regards,
> Lucas
> 
> > Signed-off-by: Robin Gong <yibin.gong@xxxxxxx>
> > ---
> > Âdrivers/dma/imx-sdma.c | 17 ++++++++++++++++-
> > Â1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c index
> > b4ec2d2..5973489 100644
> > --- a/drivers/dma/imx-sdma.c
> > +++ b/drivers/dma/imx-sdma.c
> > @@ -298,6 +298,15 @@ struct sdma_context_data {
> > > Â	u32ÂÂscratch7;
> > Â} __attribute__ ((packed));
> >
> > +/*
> > + * All bds in one transfer should be consitent on SDMA. To easily
> > +follow it,just
> > + * set the dma pool size as the enough bds. For example, in dmatest
> > +case, the
> > + * max 20 bds means the max for single transfer is NUM_BD *
> > +SDMA_BD_MAX_CNT = 20
> > + * * 65535 = ~1.28MB. 20 bds supposed to be enough basically.If it's
> > +still not
> > + * enough in some specific cases, enlarge it here.Warning message
> > +would also
> > + * appear if the bd numbers is over than 20.
> > + */
> > +#define NUM_BD 20
> >
> > Âstruct sdma_engine;
> >
> > @@ -1273,7 +1282,7 @@ static int sdma_alloc_chan_resources(struct
> > dma_chan *chan)
> > > Â		goto disable_clk_ahb;
> >
> > > Â	sdmac->bd_pool = dma_pool_create("bd_pool", chan->device->dev,
> > > -				sizeof(struct sdma_buffer_descriptor),
> > > +				NUM_BD * sizeof(struct sdma_buffer_descriptor),
> > > Â				32, 0);
> >
> > > Â	return 0;
> > @@ -1314,6 +1323,12 @@ static struct sdma_desc
> > *sdma_transfer_init(struct sdma_channel *sdmac,
> > Â{
> > > Â	struct sdma_desc *desc;
> >
> > > +	if (bds > NUM_BD) {
> > > +		dev_err(sdmac->sdma->dev, "%d bds exceed the max %d\n",
> > > +			bds, NUM_BD);
> > > +		goto err_out;
> > > +	}
> > +
> > > Â	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> > > Â	if (!desc)
> > > Â		goto err_out;