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SubjectRE: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds for one transfer
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Hello Lucas,
Any comment for my reply?

> -----Original Message-----
> From: Robin Gong
> Sent: 2018年7月25日 9:25
> To: 'Lucas Stach' <l.stach@pengutronix.de>; vkoul@kernel.org;
> dan.j.williams@intel.com; s.hauer@pengutronix.de; linux@armlinux.org.uk
> Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org
> Subject: RE: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds for one
> transfer
>
> > -----Original Message-----
> > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > Sent: 2018年7月24日 17:22
> > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > dan.j.williams@intel.com; s.hauer@pengutronix.de;
> > linux@armlinux.org.uk
> > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v3 3/3] dmaengine: imx-sdma: allocate max 20 bds
> > for one transfer
> >
> > Am Montag, den 23.07.2018, 13:55 +0000 schrieb Robin Gong:
> > > > -----Original Message-----
> > > > From: Lucas Stach [mailto:l.stach@pengutronix.de]
> > > > Sent: 2018年7月23日 18:54
> > > > To: Robin Gong <yibin.gong@nxp.com>; vkoul@kernel.org;
> > > > dan.j.williams@intel.com; s.hauer@pengutronix.de;
> > > > linux@armlinux.or g.uk
> > > > Cc: dmaengine@vger.kernel.org; dl-linux-imx <linux-imx@nxp.com>;
> > > > kernel@pengutronix.de; linux-arm-kernel@lists.infradead.org;
> > > > linux-kernel@vger.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?
> > >
> > > Yes, BDs should be contiguous  one by one in memory.
> >
> > Okay, but this isn't what the code change does. By increasing the size
> > parameter of the dma pool you just allocate 20 times as much memory as
> > needed for each BD. So actually the BDs end up being very non-
> > contiguous in memory as there are now holes of 19 BD sizes between the
> start of each BD.
> Please notice only allocate bds memory from dma pool one time even in multi
> bds.
> That's different with the common use case that allocate memory from dma
> pool everytime for every bd. Why do this is to make sure all bd memory is
> contiguous for single transfer whatever single bd or multi-bds, since two call
> dma_pool_alloc() can't promise the address is contiguous especially for multi
> thread case such as dmatest 'threads_per_chan = 5'. You can change to '
> norandom=true ' and ' test_buf_size = 163840' in dmatest.c to look what issue
> coming without this patch.
> >
> > So something isn't right with this change.
> I think this patch is the easy way to resolve the bd contiguous issue, but the
> cost is to allocate more dma pool memory which may not used.
> >
> > Regards,
> > Lucas
> >
> > > >
> > > > 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@nxp.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;
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