linux-kernel - Re: [PATCH] dmaengine: bcm2835: Fix polling for completion of DMA with interrupts masked.

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Message-ID: <20160607052405.GC16910@localhost>
Date:	Tue, 7 Jun 2016 10:54:05 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Eric Anholt <eric@...olt.net>
Cc:	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>,
	Lee Jones <lee@...nel.org>, dmaengine@...r.kernel.org
Subject: Re: [PATCH] dmaengine: bcm2835: Fix polling for completion of DMA
 with interrupts masked.

On Mon, Jun 06, 2016 at 10:33:18AM -0700, Eric Anholt wrote:
> Vinod Koul <vinod.koul@...el.com> writes:
> 
> > On Fri, Jun 03, 2016 at 07:29:11PM -0700, Eric Anholt wrote:
> >> The tx_status hook is supposed to be safe to call from interrupt
> >> context, but it wouldn't ever return completion for the last transfer,
> >> meaning you couldn't poll for DMA completion with interrupts masked.
> >
> > and why is that?
> 
> Maybe this was poorly worded.  How about:
> 
> The tx_status hook is supposed to be safe to call from interrupt
> context.  However, the current transfer currently only gets marked
> complete by the IRQ handler, so if interrupts were masked then polling
> for completion would never finish.

Sound better :)

> 
> >> This fixes IRQ handling for bcm2835's DSI1, which requires using the
> >> DMA engine to write its registers due to a bug in the AXI bridge.
> >> 
> >> Signed-off-by: Eric Anholt <eric@...olt.net>
> >> ---
> >>  drivers/dma/bcm2835-dma.c | 24 +++++++++++++++++++-----
> >>  1 file changed, 19 insertions(+), 5 deletions(-)
> >> 
> >> diff --git a/drivers/dma/bcm2835-dma.c b/drivers/dma/bcm2835-dma.c
> >> index 6149b27c33ad..320461c578e3 100644
> >> --- a/drivers/dma/bcm2835-dma.c
> >> +++ b/drivers/dma/bcm2835-dma.c
> >> @@ -570,16 +570,16 @@ static enum dma_status bcm2835_dma_tx_status(struct dma_chan *chan,
> >>  	struct virt_dma_desc *vd;
> >>  	enum dma_status ret;
> >>  	unsigned long flags;
> >> +	u32 residue;
> >>  
> >>  	ret = dma_cookie_status(chan, cookie, txstate);
> >> -	if (ret == DMA_COMPLETE || !txstate)
> >> +	if (ret == DMA_COMPLETE)
> >
> > Why do you change this? txstate can be NULL, so no point calculating reside
> > for those cases
> 
> The point was to go into the "Calculate where we're at in our current
> DMA (if the current DMA is the one we're asking about status for)" path,
> so that we could note when the DMA is complete even when there's no
> txstate passed in.

Can you explain what you mean by current DMA!

The claulation is always done for 'descriptor' represnted by the cookie. So
it doesnt not matter...!

Thanks
-- 
~Vinod

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