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

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Date:	Mon, 06 Jun 2016 23:10:38 -0700
From:	Eric Anholt <eric@...olt.net>
To:	Vinod Koul <vinod.koul@...el.com>
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.

Vinod Koul <vinod.koul@...el.com> writes:

> 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...!

By current I mean the current descriptor that has been submitted to the
hardware, in bcm2835_chan->desc.

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