linux-kernel - Re: [PATCH 09/46] dmaengine: cppi41: use dmaenginem_async_device

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Date:   Mon, 6 Aug 2018 11:28:47 +0800
From:   Huang Shijie <sjhuang@...vatar.ai>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     vkoul@...nel.org, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, dave.jiang@...el.com,
        radhey.shyam.pandey@...inx.com, appana.durga.rao@...inx.com,
        jmkrzyszt@...il.com, gomonovych@...il.com, keescook@...omium.org,
        horms+renesas@...ge.net.au, geert+renesas@...der.be,
        shawnguo@...nel.org, baoyou.xie@...aro.org,
        michal.simek@...inx.com, baohua@...nel.org,
        ludovic.desroches@...rochip.com, linus.walleij@...aro.org,
        david.brown@...aro.org
Subject: Re: [PATCH 09/46] dmaengine: cppi41: use
 dmaenginem_async_device_register to simplify the code

On Fri, Aug 03, 2018 at 10:55:25AM +0300, Peter Ujfalusi wrote:
> 
> 
> On 2018-08-03 10:19, Huang Shijie wrote:
> > Use dmaenginem_async_device_register to simplify the code:
> >    remove dma_async_device_unregister
> > 
> > Signed-off-by: Huang Shijie <sjhuang@...vatar.ai>
> > ---
> >  drivers/dma/ti/cppi41.c | 7 ++-----
> >  1 file changed, 2 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/dma/ti/cppi41.c b/drivers/dma/ti/cppi41.c
> > index 1497da367710..d2998a19ed2e 100644
> > --- a/drivers/dma/ti/cppi41.c
> > +++ b/drivers/dma/ti/cppi41.c
> > @@ -1096,21 +1096,19 @@ static int cppi41_dma_probe(struct platform_device *pdev)
> >  		goto err_chans;
> >  	cdd->irq = irq;
> >  
> > -	ret = dma_async_device_register(&cdd->ddev);
> > +	ret = dmaenginem_async_device_register(&cdd->ddev);
> >  	if (ret)
> >  		goto err_chans;
> >  
> >  	ret = of_dma_controller_register(dev->of_node,
> >  			cppi41_dma_xlate, &cpp41_dma_info);
> >  	if (ret)
> > -		goto err_of;
> > +		goto err_chans;
> >  
> >  	pm_runtime_mark_last_busy(dev);
> >  	pm_runtime_put_autosuspend(dev);
> >  
> >  	return 0;
> > -err_of:
> > -	dma_async_device_unregister(&cdd->ddev);
> >  err_chans:
> >  	deinit_cppi41(dev, cdd);
> >  err_init_cppi:
> > @@ -1132,7 +1130,6 @@ static int cppi41_dma_remove(struct platform_device *pdev)
> >  		dev_err(&pdev->dev, "%s could not pm_runtime_get: %i\n",
> >  			__func__, error);
> >  	of_dma_controller_free(pdev->dev.of_node);
> > -	dma_async_device_unregister(&cdd->ddev);
> 
> If I read the code right then this is not safe.
I read the code again, and find it is okay.

> We would have deinitalized cppi41 driver which is not functional, but we
> will still have the dma device registered and if a channel is requested
> we will have kernel crash.
We cannot succeed to request a channel when the drv->remove() is called.

Please see __device_release_driver:
  ---------------------------------------------------------------------
		if (dev->bus && dev->bus->remove)
			dev->bus->remove(dev);
		else if (drv->remove)
			drv->remove(dev);

		device_links_driver_cleanup(dev);
		dma_deconfigure(dev);

		devres_release_all(dev);    ============> Devres release
  ---------------------------------------------------------------------

For the DMA engine driver, there is only one case which will calls drv->remove():
        Use the rmmod(or modprobe -r).

We do not use the device_link_add API for DMA engines.
And we not manually call the device_release_driver() for DMA engines.

But when we use the rmmod, the module state will be MODULE_STATE_GOING.
In the find_candidate(), dma_chan_get() will fail.
And we cannot get a channel.

Please correct me if I am wrong :)

Thanks
Huang Shijie