linux gpio

Jürgen Lambrecht J.Lambrecht at
Mon Apr 4 07:44:08 EDT 2011

On 04/03/2011 09:36 AM, Jim Cromie wrote:
> 2011/3/29 Jürgen Lambrecht<J.Lambrecht at>:
>> Hello,
>> I am new to linux (coming from eCos).
>> My backlight driver does not start up because the reset gpio pin is not
>> correct specified.
>> How does gpio work in Linux?
> theres no single way.
> the new way is in drivers/gpio/* , implemented in gpiolib.c
> defined in #include<linux/gpio.h>
thanks, i'll use that to write a gpio driver for our own hw.
> there are also char drivers, with which Im more familiar.
> ./char/nsc_gpio.c
> ./char/pc8736x_gpio.c
> ./char/scx200_gpio.c
>> I have 2 platforms that use the same backlight driver, under arm, for freescale imx: imx31_3ds and imx27_3ds.
>> I don't think that matters, I just need to understand the linux gpio.
>> The imx31 platform gives the number 88 as gpio number to the backlight
>> driver.
>> That number comes out of 'iomux-mx3.h'.
>> In 'iomux-mx27.h' the GPIO's are specified in an other way, the
>> "hardware" way, specifying bank and port. I think Linux GPIO needs a
>> sort of general gpio number.
> The problem here is that board IDs are somewhat arbitrary,
> whereas driver IDs are predictable.  Take soekris 4801 as an example;
> it has gpio on the CPU-SOC (scx200_gpio), and on a peripheral chip
> (pc8736x_gpio)
> The board uses CPU pins for Leds (and reset button I think),
> but the GPIO exposed to connector is on pc87366 chip.
> The char drivers use major device numbers to select the chip&  driver,
> and minor numbers to select the bank/pin on the chip.
> THis lets users create device files with suitable names / numbers.
ok, interesting
>> But how do I calculate that number out of the datasheet?
>> In the imx31 datasheet, I cannot find that number 88 (or the other
>> numbers in 'iomux-mx3.h').
> what driver supports your boards ?
> is it a new gpio driver ?
It is ok now: i found it in arch/arm/plat-mxc/include/mach/gpio.h.
Thanks, the backlight driver starts up now.
Now I'm looking why udev (or mdev) does not create a /dev/fb0.. reading 
about it..

Kind regards,

Jürgen Lambrecht
R&D Associate
Tel: +32 (0)51 303045    Fax: +32 (0)51 310670
Televic Rail NV - Leo Bekaertlaan 1 - 8870 Izegem - Belgium
Company number 0825.539.581 - RPR Kortrijk

More information about the Kernelnewbies mailing list