linux-kernel - Re: [PATCH] Allow to use DMA_CTRL_REUSE flag for all channel types

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Message-ID: <1493738176.25985.23.camel@synopsys.com>
Date:   Tue, 2 May 2017 15:16:18 +0000
From:   Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:     "vinod.koul@...el.com" <vinod.koul@...el.com>
CC:     "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "Eugeniy.Paltsev@...opsys.com" <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH] Allow to use DMA_CTRL_REUSE flag for all channel types

Hi Vinod,

On Mon, 2017-05-01 at 11:21 +0530, Vinod Koul wrote:
> On Fri, Apr 28, 2017 at 04:37:46PM +0300, Eugeniy Paltsev wrote:
> > In the current implementation dma_get_slave_caps is used to check
> > state of descriptor_reuse option. But dma_get_slave_caps includes
> > check if the channel supports slave transactions.
> > So DMA_CTRL_REUSE flag can be set (even for MEM-TO-MEM tranfers)
> > only if channel supports slave transactions.
> > 
> > Now we can use DMA_CTRL_REUSE flag for all channel types.
> > Also it allows to test reusing mechanism with simply mem-to-mem dma
> > test.
> 
> We do not want to allow that actually. Slave is always treated as a
> special
> case, so resue was allowed.
> 
> With memcpy the assumptions are different and clients can do reuse.

Could you please clarify why don't we want to allow use DMA_CTRL_REUSE
for mem-to-mem transfers?

Reusing of mem-to-mem (MEMCPY and DMA_SG) descriptors will work fine on
virt-dma based drivers.

Anyway the current implementation behaviour is quite strange:
If channel supports *slave* transfers DMA_CTRL_REUSE can be set to
slave and *mem-to-mem* transfers.

And, of course, we can pass DMA_CTRL_REUSE flag to device_prep_dma_sg
or device_prep_dma_memcpy directly without checks.

> > 
> > Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
> > ---
> >  include/linux/dmaengine.h | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> > index 5336808..92cf8b0 100644
> > --- a/include/linux/dmaengine.h
> > +++ b/include/linux/dmaengine.h
> > @@ -1376,11 +1376,7 @@ static inline int dma_get_slave_caps(struct
> > dma_chan *chan,
> >  
> >  static inline int dmaengine_desc_set_reuse(struct
> > dma_async_tx_descriptor *tx)
> >  {
> > -	struct dma_slave_caps caps;
> > -
> > -	dma_get_slave_caps(tx->chan, &caps);
> > -
> > -	if (caps.descriptor_reuse) {
> > +	if (tx->chan->device->descriptor_reuse) {
> >  		tx->flags |= DMA_CTRL_REUSE;
> >  		return 0;
> >  	} else {
> > -- 
> > 2.9.3
> > 
> > --
-- 
 Eugeniy Paltsev