linux-kernel - RE: [PATCH v5 3/3] dmaengine: xilinx_dma: Fix race condition in the driver for multiple descriptor scenario

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Date:   Fri, 13 Jan 2017 06:33:33 +0000
From:   Appana Durga Kedareswara Rao <appana.durga.rao@...inx.com>
To:     Vinod Koul <vinod.koul@...el.com>
CC:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "michal.simek@...inx.com" <michal.simek@...inx.com>,
        Soren Brinkmann <sorenb@...inx.com>,
        "moritz.fischer@...us.com" <moritz.fischer@...us.com>,
        "laurent.pinchart@...asonboard.com" 
        <laurent.pinchart@...asonboard.com>,
        "luis@...ethencourt.com" <luis@...ethencourt.com>,
        "Jose.Abreu@...opsys.com" <Jose.Abreu@...opsys.com>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: RE: [PATCH v5 3/3] dmaengine: xilinx_dma: Fix race condition in the
 driver for multiple descriptor scenario

Hi Vinod,
	
	Thanks for the review...

[Snip]
> > > Btw how and when does DMA stop, assuming it is circular it never
> > > would, isn't there a valid/stop flag associated with a descriptor
> > > which tells DMA engine what to do next
> >
> > There are two registers that controls the DMA transfers.
> > Current descriptor and tail descriptor register.
> > When current descriptor reaches tail descriptor dma engine will pause.
> >
> > When reprogramming the tail descriptor the DMA engine will starts fetching
> descriptors again.
> >
> > But with the existing driver flow if we reprogram the tail descriptor
> > The tail descriptor next descriptor field is pointing to an invalid
> > location Causing data corruption...
> 
> So the solution is..?

This patch.
I mean if we have a set of descriptors in chain (in circular manner last descriptor points to first descriptor)
It always points to valid descriptors. 

Will update the patch commit description in the next version...

> 
> > > Btw there is something wrong with your MUA perhaps line are
> > > titlecased for no reason. This is typically behavious of non linux
> > > tool which may not be great tool for this work.
> >
> > Thanks for pointing it out.
> > I usually replies from outlook from a windows machine.
> > Will check with others in my team how they configured their mail client.
> 
> Yeah that isnt right tool for the job. See Documentation/process/email-
> clients.rst
> 
> FWIW, I use mutt, vim as editor with exchange servers, seems to work well for
> me now.

Thanks for pointing it out will go through it.
Will install mutt in my Linux machine will start replying from that

Regards,
Kedar.

> 
> --
> ~Vinod