linux-kernel - RE: [PATCH 08/09] ARM: s3c2440: gta02: Request usb pullup pin before using it

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Message-id: <031101cbdc92$e6c44730$b44cd590$%kim@samsung.com>
Date:	Mon, 07 Mar 2011 15:42:56 +0900
From:	Kukjin Kim <kgene.kim@...sung.com>
To:	'Lars-Peter Clausen' <lars@...afoo.de>
Cc:	'Ben Dooks' <ben-linux@...ff.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: RE: [PATCH 08/09] ARM: s3c2440: gta02: Request usb pullup pin before
 using it

Lars-Peter Clausen wrote:
> 
> On 02/28/2011 11:44 AM, Kukjin Kim wrote:
> > Lars-Peter Clausen wrote:
> >>
> >> On 02/28/2011 07:16 AM, Kukjin Kim wrote:
> >>> Lars-Peter Clausen wrote:
> >>>>
> >>>> Request the gpio pin used to control the usb pullup before using it
to
> >>> avoid
> >>>> a
> >>>> runtime warning about an auto-requested gpio.
> >>>>
> >>>> Signed-off-by: Lars-Peter Clausen <lars@...afoo.de>
> >>>> ---
> >>>>  arch/arm/mach-s3c2440/mach-gta02.c |   17 +++++++++++++++--
> >>>>  1 files changed, 15 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/arch/arm/mach-s3c2440/mach-gta02.c
> >>> b/arch/arm/mach-s3c2440/mach-
> >>>> gta02.c
> >>>> index 1396639..94456fa 100644
> >>>> --- a/arch/arm/mach-s3c2440/mach-gta02.c
> >>>> +++ b/arch/arm/mach-s3c2440/mach-gta02.c
> >>>> @@ -451,11 +451,11 @@ static void gta02_udc_command(enum
> > s3c2410_udc_cmd_e
> >>>> cmd)
> >>>>  	switch (cmd) {
> >>>>  	case S3C2410_UDC_P_ENABLE:
> >>>>  		pr_debug("%s S3C2410_UDC_P_ENABLE\n", __func__);
> >>>> -		gpio_direction_output(GTA02_GPIO_USB_PULLUP, 1);
> >>>> +		gpio_set_value(GTA02_GPIO_USB_PULLUP, 1);
> >>>
> >>> How about following instead?
> >>> 	gpio_request(GTA02_GPIO_USB_PULLUP, "USB_PULLUP");
> >>> 	gpio_direction_output(GTA02_GPIO_USB_PULLUP, 1);
> >>> 	gpio_free(GTA02_GPIO_USB_PULLUP);
> >>>
> >>
> >> I don't think that is a good idea. This gpio should really be reserved
for
> >> the
> >> udc driver. If it is freed again, it could be requested from someone
else
> >> which
> >> could lead to undefined behaviour.
> >>
> > Yes right, but I mean the board designer already knows the usage of
> > regarding GPIOs on his board.
> > So why do we really need gpio_request for it?...
> 
> Well, for one because it's part of the gpio-api. You should not call any
> other
> gpio functions on a pin unless you've successfully requested that pin.
> On the other hand this helps debugging and ensures that the same gpio is
not
> used by two drivers accident. For example it is also possible to request
> gpios
> from userspace using the gpio sysfs. So by keeping the gpio requested it
wont
> be possible to request it from userspace by accident. And the gpio will
also
> be
> listed in the gpio debugfs file, which can be helpful for debugging as
well.
> 
> - Lars

Hi Lars,

Ok, will apply :)
Thanks.

Best regards,
Kgene.
--
Kukjin Kim <kgene.kim@...sung.com>, Senior Engineer,
SW Solution Development Team, Samsung Electronics Co., Ltd.

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