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

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Date:	Tue, 24 Feb 2015 22:56:23 +0900
From:	Mark Brown <broonie@...nel.org>
To:	Stanimir Varbanov <stanimir.varbanov@...aro.org>
Cc:	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 Tue, Feb 24, 2015 at 03:00:03PM +0200, Stanimir Varbanov wrote:

> +static void spi_qup_dma_done(void *data)
> +{
> +	struct spi_qup *qup = data;
> +
> +	if (atomic_dec_and_test(&qup->dma_outstanding))
> +		complete(&qup->done);
> +}

I'm finding it hard to be thrilled about the use of atomics for
synchronization (they're just generally hard to work with) and...

> +	cookie = dmaengine_submit(desc);
> +	ret = dma_submit_error(cookie);
> +	if (ret)
> +		return ret;

> +	atomic_inc(&qup->dma_outstanding);

..don't we have two potential races here: one if somehow the DMA manages
to complete prior to the atomic_inc() (unlikely but that's what race
conditions are all about really) and one if we are issuing multiple DMAs
and the early ones complete before the later ones are issued?

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