linux-kernel - Re: [PATCH v2] spi: qup: Add DMA capabilities

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Date:	Fri, 27 Feb 2015 16:46:17 +0200
From:	Stanimir Varbanov <svarbanov@...sol.com>
To:	Mark Brown <broonie@...nel.org>
CC:	Stanimir Varbanov <stanimir.varbanov@...aro.org>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	devicetree@...r.kernel.org, linux-spi@...r.kernel.org,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Kumar Gala <galak@...eaurora.org>,
	Andy Gross <agross@...eaurora.org>,
	Sagar Dharia <sdharia@...eaurora.org>,
	Daniel Sneddon <dsneddon@...eaurora.org>
Subject: Re: [PATCH v2] spi: qup: Add DMA capabilities

On 02/26/2015 04:33 AM, Mark Brown wrote:
> On Tue, Feb 24, 2015 at 06:08:54PM +0200, Stanimir Varbanov wrote:
> 
>> yes, there is a potential race between atomic_inc and dma callback. I
>> reordered these calls to save few checks, and now it returns to me.
> 
>> I imagine few options here:
> 
>>  - reorder the dmaengine calls and atomic operations, i.e.
>> call atomic_inc for rx and tx channels before corresponding
>> dmaengine_submit and dmaengine_issue_pending.
> 
>>  - have two different dma callbacks and two completions and waiting for
>> the two.
> 
>>  - manage to receive only one dma callback, i.e. the last transfer in
>> case of presence of the rx_buf and tx_buf at the same time.
> 
>>  - let me see for better solution.
> 
> Any solution which doesn't make use of atomics is likely to be better,
> as I said they are enormously error prone.  A more common approach is a
> single completion triggering on the RX (for RX only or bidirectional
> transfers) or TX if that's the only thing active.  For most hardware you
> can just use the RX to manage completion since it must of necessity
> complete at the same time as or later than the transmit side, transmit
> often completes early since the DMA completes when the FIFO is full not
> when the data is on the wire.
> 

yep, that's what I wanted to express in third option above.

-- 
regards,
Stan
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