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

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Message-ID: <20170510035507.GW6263@localhost>
Date:   Wed, 10 May 2017 09:25:07 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.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>,
        "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

On Tue, May 02, 2017 at 03:16:18PM +0000, Eugeniy Paltsev wrote:
> 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.

Precisely, the client does not know if you have a virt-dma or some other
kind if implementation

For them they see a channel and use it!

> 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.

Yeah thats bad, do send a patch to forbid that..

-- 
~Vinod