[U-Boot] [PATCH 1/2] dm: gpio: mxc: fix mxc_gpio_bank_get_value

[Stefano Babic]

Hi Peng,

On 17/03/2016 02:44, Peng Fan wrote:
> On Wed, Mar 16, 2016 at 09:48:24AM -0300, Fabio Estevam wrote:
>> Hi Peng,
>>
>> On Tue, Mar 15, 2016 at 10:27 PM, Peng Fan <van.freenix at gmail.com> wrote:
>>
>>> If set the SION bit, we need to change the pinmux settings in device tree,
>>> however device tree are introduced from linux kernel. I would not like
>>> to change the pinmux setting value.
>>>
>>> So I choose to use data register but not PSR register.
>>
>> Haven't checked the kernel driver, but just curious: how does the
>> kernel handle this?
> 
> bgc->gc.get = bgpio_get;
> 
> The get method:
> * @get: returns value for signal "offset"; for output signals this
> *      returns either the value actually sensed, or zero
> 
> 138 static int bgpio_get(struct gpio_chip *gc, unsigned int gpio)
> 139 {
> 140         struct bgpio_chip *bgc = to_bgpio_chip(gc);
> 141
> 142         return !!(bgc->read_reg(bgc->reg_dat) & bgc->pin2mask(bgc, gpio));
> 143 }
> 
> The gpio subsystem use reg_data. The reg_data is GPIO_PSR for gpio-mxc.c:
> 
> 450         err = bgpio_init(&port->bgc, &pdev->dev, 4,
> 451                          port->base + GPIO_PSR,
> 452                          port->base + GPIO_DR, NULL,
> 453                          port->base + GPIO_GDIR, NULL, 0);
> 
> So we know that kernel gpio-mxc use GPIO_PSR for reading data, but
> as the comments for get method "for output signals this returns
> either the value actually sensed, or zero", if no SION bit set, reading
> PSR will return 0 with direction output.
> 

There was already a debate in the past about this point. You can take a
look at the commit:

commit 5dafa4543c399d329c7b01df1afa98437861cac0
Author: Benoît Thébaudeau <benoit.thebaudeau at advansee.com>
Date:   Mon Aug 20 10:55:41 2012 +0000

    mxc: Make gpio_get_value() use PSR


There were good reasons to switch to PSR (mxc-gpio used DR before) - we
cannot switch any time back to solve a single issue.

I would like to remain compliant with Linux - your change breaks also
boards that rely on this behavior.


> But in U-Boot, the regulator driver needs to get the value when direction is
> configured as output, we can use data register here or configure SION in pinmux.
> But I prefer using data register here.
> 

Best regards,
Stefano

-- 
=====================================================================
DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-53 Fax: +49-8142-66989-80 Email: sbabic at denx.de
=====================================================================