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

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Date:	Wed, 22 Jul 2015 15:26:01 +0100
From:	Alex Smith <alex.smith@...tec.com>
To:	Vinod Koul <vinod.koul@...el.com>
CC:	Julia Lawall <julia.lawall@...6.fr>, <Zubair.Kakakhel@...tec.com>,
	<dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: question about drivers/dma/dma-jz4780.c

On 21/07/2015 05:15, Vinod Koul wrote:
> On Mon, Jul 20, 2015 at 10:28:14AM +0200, Julia Lawall wrote:
>> On Mon, 20 Jul 2015, Alex Smith wrote:
>>
>>> 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?
> Not just users but from a device management you are ensuring that your
> device is not able to send any more interrupts and also your tasklet is not
> spawned further. For this it is neccessary that devm_irq_free be called
> explcitly by devices
>>>>
>>>> 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.
> Are you ensuring that device can no longer sent interrupts and all instances
> of tasklet running or either completed are terminated and no further tasklet
> can be spawned?

Hi Vinod,

If I understand correctly, when remove() is called, there should be no 
more users of the DMA controller, enforced by the module reference count.

Wouldn't that guarantee that there are no more transactions running and 
therefore no chance of an interrupt from the controller or a tasklet 
still running?

Thanks,
Alex
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