linux-kernel - Re: [PATCH v2 2/4] input: keyboard: tegra: use devm

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Date:	Sat, 5 Jan 2013 15:18:58 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Laxman Dewangan <ldewangan@...dia.com>
Cc:	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Stephen Warren <swarren@...dia.com>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] input: keyboard: tegra: use devm_* for resource
 allocation

On Sat, Jan 05, 2013 at 04:50:58PM +0530, Laxman Dewangan wrote:
> HI Dmitry,
> Thanks for quick review.
> 
> I will take care of your comment in next version. Some have my answer.
> 
> 
> On Saturday 05 January 2013 01:36 PM, Dmitry Torokhov wrote:
> >Hi Laxman,
> >
> >On Sat, Jan 05, 2013 at 01:15:08PM +0530, Laxman Dewangan wrote:
> >>Use devm_* for memory, clock, input device allocation. This reduces
> >>code for freeing these resources.
> 
> >>  	err = tegra_kbd_setup_keymap(kbc);
> >>-	if (err) {
> >>+	if (err < 0) {
> >Why is this change? As far as I can see tegra_kbd_setup_keymap() never
> >returns positive values.
> 
> Ok, mostly errors are in negative and hence this change, I will
> revert it and will keep original.
> 
> >
> >>  		dev_err(&pdev->dev, "failed to setup keymap\n");
> >>-		goto err_put_clk;
> >>+		return err;
> >>  	}
> >>  	__set_bit(EV_REP, input_dev->evbit);
> >>@@ -790,15 +784,15 @@ static int tegra_kbc_probe(struct platform_device *pdev)
> >>  	err = request_irq(kbc->irq, tegra_kbc_isr,
> >>  			  IRQF_NO_SUSPEND | IRQF_TRIGGER_HIGH, pdev->name, kbc);
> >>-	if (err) {
> >>+	if (err < 0) {
> >Neither request_irq(). BTW, why not devm_request_irq?
> 
> I understand from Mark B on different patches that using
> devm_request_irq() can create race condition when removing device.
> Interrupt can occur when device resource release is in process and
> so it can cause isr call which can use the freed pointer.
> devm_request_irq() should be avoided.

devm_request_irq() has a potential of creating a race condition, but it
depents on the driver. In this particular case tegra driver ensures that
interrupts are inhibited when input device is unregistered by providing
tegra_kbc_close() method, so in this particular case it is safe to
use devm_request_irq().

Also, when using managed input devices, the unregistering and final
freeing is a 2-step process, so even in absence of close() method, if
initialization sequence was:

	devm_input_allocate_device()
	...
	devm_request_irq()
	...
	input_unregister_device()

then order of freeing resources (behind the scenes) will be

	devm_input_device_unregister();
	/* input device is still present in memory and can
	 * handle input_event() calls.
	 */
	free_irq();
	devm_input_device_release();

So using managed request_irq() _together_ with managed input devices is
OK.

Thanks.

-- 
Dmitry
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