linux-kernel - Re: [PATCH] xen: check return value of xenbus

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Message-ID: <561FEE60.7080308@citrix.com>
Date:	Thu, 15 Oct 2015 19:20:16 +0100
From:	Julien Grall <julien.grall@...rix.com>
To:	Insu Yun <wuninsu@...il.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<huaixin.chx@...baba-inc.com>, <linux-input@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Taesoo Kim <taesoo@...ech.edu>,
	Yeongjin Jang <yeongjin.jang@...ech.edu>,
	"Yun, Insu" <insu@...ech.edu>
Subject: Re: [PATCH] xen: check return value of xenbus_printf

Hi Insu,

On 15/10/15 19:12, Insu Yun wrote:
> 
> 
> On Thu, Oct 15, 2015 at 12:40 PM, David Vrabel <david.vrabel@...rix.com
> <mailto:david.vrabel@...rix.com>> wrote:
> 
>     On 15/10/15 17:25, Insu Yun wrote:
>     > Internally, xenbus_printf uses memory allocation, so it can be failed in
>     > memory pressure.Therefore, xenbus_printf's return should be checked
>     > and properly handled.
>     [...]
>     > --- a/drivers/input/misc/xen-kbdfront.c
>     > +++ b/drivers/input/misc/xen-kbdfront.c
>     > @@ -129,8 +129,11 @@ static int xenkbd_probe(struct xenbus_device *dev,
>     >
>     >       if (xenbus_scanf(XBT_NIL, dev->otherend, "feature-abs-pointer", "%d", &abs) < 0)
>     >               abs = 0;
>     > -     if (abs)
>     > -             xenbus_printf(XBT_NIL, dev->nodename, "request-abs-pointer", "1");
>     > +     if (abs) {
>     > +             ret = xenbus_printf(XBT_NIL, dev->nodename, "request-abs-pointer", "1");
>     > +             if (ret)
> 
>     > +                     pr_warning("xenkbd: can't request abs-pointer");
> 
> 
> This error handling is from other code .
> I am not sure that it is right error handling.
>  
> 
> 
>     I think you want abs = 0 here or input device will be configured as
>     absolute but the backend will supply relative coordinates.
> 
> 
> I cannot understand

If the frontend is not able to write the node "request-abs-pointer" in
the xenstore, the backend will always supply relative coordinates.

Although, as abs = 1, the frontend will be configured to handle absolute
coordinate. So the backend and frontend won't be able to understand each
other.

So you have to set abs to 0 if xebus_printf fails.

Regards,

-- 
Julien Grall
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