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

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Message-ID: <1532343252.3163.99.camel@pengutronix.de>
Date:   Mon, 23 Jul 2018 12:54:12 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Robin Gong <yibin.gong@....com>, vkoul@...nel.org,
        dan.j.williams@...el.com, s.hauer@...gutronix.de,
        linux@...linux.org.uk
Cc:     dmaengine@...r.kernel.org, linux-imx@....com,
        kernel@...gutronix.de, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
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?

What do you gain by over-allocating each BD by a factor of 20?

Regards,
Lucas

> Signed-off-by: Robin Gong <yibin.gong@....com>
> ---
>  drivers/dma/imx-sdma.c | 17 ++++++++++++++++-
>  1 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 {
> >  	u32  scratch7;
>  } __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
>  
>  struct sdma_engine;
>  
> @@ -1273,7 +1282,7 @@ static int sdma_alloc_chan_resources(struct dma_chan *chan)
> >  		goto disable_clk_ahb;
>  
> >  	sdmac->bd_pool = dma_pool_create("bd_pool", chan->device->dev,
> > -				sizeof(struct sdma_buffer_descriptor),
> > +				NUM_BD * sizeof(struct sdma_buffer_descriptor),
> >  				32, 0);
>  
> >  	return 0;
> @@ -1314,6 +1323,12 @@ static struct sdma_desc *sdma_transfer_init(struct sdma_channel *sdmac,
>  {
> >  	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;
> > +	}
> +
> >  	desc = kzalloc((sizeof(*desc)), GFP_NOWAIT);
> >  	if (!desc)
> >  		goto err_out;