linux-kernel - Re: A proposal to check the device in generic way

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Date:	Mon, 11 Mar 2013 13:26:01 +0100 (CET)
From:	Guennadi Liakhovetski <g.liakhovetski@....de>
To:	"Shevchenko, Andriy" <andriy.shevchenko@...el.com>
cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	"Koul, Vinod" <vinod.koul@...el.com>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Russell King <linux@....linux.org.uk>
Subject: Re: A proposal to check the device in generic way

Hi Andriy

(adding Russell to CC)

On Mon, 11 Mar 2013, Shevchenko, Andriy wrote:

> Hello.
> 
> Currently in linux-next we have the following things:
> 
> $ git grep -n 'chan->device->dev->driver' drivers/dma/
> 
> drivers/dma/amba-pl08x.c:1594:  if (chan->device->dev->driver !=
> &pl08x_amba_driver.drv)
> drivers/dma/dmaengine.c:190:    return chan->device->dev->driver->owner;
> drivers/dma/edma.c:609: if (chan->device->dev->driver ==
> &edma_driver.driver) {
> drivers/dma/omap-dma.c:654:     if (chan->device->dev->driver ==
> &omap_dma_driver.driver) {
> drivers/dma/pl330.c:2374:       if (chan->device->dev->driver !=
> &pl330_driver.drv)
> drivers/dma/sa11x0-dma.c:1080:  if (chan->device->dev->driver ==
> &sa11x0_dma_driver.driver) 
> 
> I think it's a non-generic way to check which driver provides a channel
> into filter function. First of all, I don't get why that comparison goes
> as deep as driver structure. Isn't clearer to check chan->device->dev
> against the struct dev passed in the custom parameter structure? Like:
> 
> struct filter_params {
>  struct dev *dev;
>  void *param;
> };

I don't think you always know which DMA device you want to use with this 
DMA client - sometimes there are several DMA engine devices, that can be 
used with your DMA client, or even if it's only one, you don't necessarily 
have a pointer to it in your DMA client.

Thanks
Guennadi

> bool filter_fn(struct dma_chan *chan, void *fparams)
> {
>   struct filter_params *p = fparams;
>   if (chan->device->dev != p->dev)
>    return false;
>  ...
> }
> 
> In case my idea has a right to live, what about to move such check inside
> DMA engine code?
> 
> Opinions, comments?
> 
> Earlier I tried to discuss this with Arnd here: http://www.spinics.net/lists/arm-kernel/msg220716.html

---
Guennadi Liakhovetski, Ph.D.
Freelance Open-Source Software Developer
http://www.open-technology.de/
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