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

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Date:	Mon, 06 Jun 2016 10:33:18 -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 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.

>> 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.

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