linux-kernel - Re: question about drivers/dma/dma-jz4780.c

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Message-ID: <55ACAEC5.7090000@imgtec.com>
Date:	Mon, 20 Jul 2015 09:18:13 +0100
From:	Alex Smith <alex.smith@...tec.com>
To:	Julia Lawall <julia.lawall@...6.fr>
CC:	<Zubair.Kakakhel@...tec.com>, <dmaengine@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: question about drivers/dma/dma-jz4780.c

On 19/07/2015 10:08, Julia Lawall wrote:
> The file drivers/dma/dma-jz4780.c has a probe function that sets up irqs
> using devm_request_irq.  The probe function then ends with:
>
> err_unregister_dev:
> 	dma_async_device_unregister(dd);
>
> err_disable_clk:
>          clk_disable_unprepare(jzdma->clk);
> 	return ret;
> }
>
> The remove function, on the other hand contains:
>
>          of_dma_controller_free(pdev->dev.of_node);
>          devm_free_irq(&pdev->dev, jzdma->irq, jzdma);
>          dma_async_device_unregister(&jzdma->dma_device);
>
> The need for calling devm_free_irq explicitly would be that it needs to
> occur before dma_async_device_unregister, to eg avoid a reference to a
> dangling pointer.  But devm_free_irq is implicitly called after the call
> to dma_async_device_unregister at the end of the probe function.  Which
> one is correct?
>
> julia

Hi,

I think the explicit devm_free_irq() here is unnecessary, as when remove 
is called there should be no remaining users of the DMA controller and 
therefore no chance of an IRQ occurring between the controller being 
unregistered and an implicit IRQ release afterwards.

I recently sent a series of fixes for this driver, I will send a new 
version with a patch to remove the unnecessary free.

Thanks,
Alex
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