linux-kernel - Re: [PATCH -next 7/8] dmaengine: xilinx_dma: Check for both idle and halted state in axidma stop

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Message-ID: <20190927135720.GA16057@lnx-nickg>
Date:   Fri, 27 Sep 2019 08:57:20 -0500
From:   Nicholas Graumann <nick.graumann@...il.com>
To:     Radhey Shyam Pandey <radheys@...inx.com>
Cc:     Vinod Koul <vkoul@...nel.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        Michal Simek <michals@...inx.com>,
        "andrea.merello@...il.com" <andrea.merello@...il.com>,
        Appana Durga Kedareswara Rao <appanad@...inx.com>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next 7/8] dmaengine: xilinx_dma: Check for both idle and
 halted state in axidma stop_transfer

On Fri, Sep 27, 2019 at 06:48:29AM +0000, Radhey Shyam Pandey wrote:
> > -----Original Message-----
> > From: Vinod Koul <vkoul@...nel.org>
> > Sent: Thursday, September 26, 2019 10:51 PM
> > To: Radhey Shyam Pandey <radheys@...inx.com>
> > Cc: dan.j.williams@...el.com; Michal Simek <michals@...inx.com>;
> > nick.graumann@...il.com; andrea.merello@...il.com; Appana Durga
> > Kedareswara Rao <appanad@...inx.com>; mcgrof@...nel.org;
> > dmaengine@...r.kernel.org; linux-kernel@...r.kernel.org
> > Subject: Re: [PATCH -next 7/8] dmaengine: xilinx_dma: Check for both idle
> > and halted state in axidma stop_transfer
> > 
> > On 05-09-19, 22:07, Radhey Shyam Pandey wrote:
> > > From: Nicholas Graumann <nick.graumann@...il.com>
> > >
> > > When polling for a stopped transfer in AXI DMA mode, in some cases the
> > > status of the channel may indicate IDLE instead of HALTED if the
> > > channel was reset due to an error.
> > >
> > > Signed-off-by: Nicholas Graumann <nick.graumann@...il.com>
> > > Signed-off-by: Radhey Shyam Pandey
> > <radhey.shyam.pandey@...inx.com>
> > > ---
> > >  drivers/dma/xilinx/xilinx_dma.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/dma/xilinx/xilinx_dma.c
> > b/drivers/dma/xilinx/xilinx_dma.c
> > > index b5dd62a..0896e07 100644
> > > --- a/drivers/dma/xilinx/xilinx_dma.c
> > > +++ b/drivers/dma/xilinx/xilinx_dma.c
> > > @@ -1092,8 +1092,9 @@ static int xilinx_dma_stop_transfer(struct
> > xilinx_dma_chan *chan)
> > >
> > >  	/* Wait for the hardware to halt */
> > >  	return xilinx_dma_poll_timeout(chan, XILINX_DMA_REG_DMASR,
> > val,
> > > -				       val & XILINX_DMA_DMASR_HALTED, 0,
> > > -				       XILINX_DMA_LOOP_COUNT);
> > > +				       val | (XILINX_DMA_DMASR_IDLE |
> > > +					      XILINX_DMA_DMASR_HALTED),
> > 
> > The condition was bitwise AND and now is OR.. ??
> 
> Ah, it should be same as before . Only _IDLE mask should be in OR.
> 
> Also on second thought to this patch- we need to describe which error
> scenario "in some cases the status of the channel may indicate IDLE
> instead of HALTED" as mentioned in commit description.
> 
> @Nick: Can you comment?
> 
In regard to the mask question, yes, this looks like a bug.
We should be AND'ing with the mask like before.

As far as the state, usually when we saw the IDLE state when invoking 
dmaengine_terminate_all on a channel that had errors. I have not
proved this, but I believe what happened was the following:

New transactions were queued when chan->err was set, causing
xilinx_dma_chan_reset to be invoked which ultimately results in the
hardware being in an IDLE state by the time xilinx_dma_terminate_all
gets around to invoking stop_transfer. At that point, stop_transfer is
going to time out waiting for the hardware to indicate it has HALTED and
ultimately will time out.


In any case, xilinx_dma_stop_transfer should be fine with the hardware
being in an IDLE state to indicate that the active transfer is stopped.
Case in point: The CDMA core also covered by this driver only has an
IDLE bit and no HALTED bit in its DMASR, and it checks for just the IDLE
bit in xilinx_cdma_stop_transfer().
> > 
> > > +				       0, XILINX_DMA_LOOP_COUNT);
> > >  }
> > >
> > >  /**
> > > --
> > > 2.7.4
> > 
> > --
> > ~Vinod