Re: [PATCH 1/3] em28xx: give up GPIO register tracking/caching

["Frank Schäfer" <fschaefer.oss@googlemail.com>]

Am 13.04.2013 19:04, schrieb Mauro Carvalho Chehab:
> Em Sat, 13 Apr 2013 17:33:28 +0200
> Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:
>
>> Am 13.04.2013 16:41, schrieb Mauro Carvalho Chehab:
>>> Em Sat, 13 Apr 2013 11:48:39 +0200
>>> Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:
>>>
>>>> The GPIO register tracking/caching code is partially broken, because newer
>>>> devices provide more than one GPIO register and some of them are even using
>>>> separate registers for read and write access.
>>>> Making it work would be too complicated.
>>>> It is also used nowhere and doesn't make sense in cases where input lines are
>>>> connected to buttons etc.
>>>>
>>>> Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
>>>> ---
>>>>  drivers/media/usb/em28xx/em28xx-cards.c |   12 ------------
>>>>  drivers/media/usb/em28xx/em28xx-core.c  |   27 ++-------------------------
>>>>  drivers/media/usb/em28xx/em28xx.h       |    6 ------
>>>>  3 Dateien geändert, 2 Zeilen hinzugefügt(+), 43 Zeilen entfernt(-)
>>> ...
>>>
>>>
>>>> @@ -231,14 +215,7 @@ int em28xx_write_reg_bits(struct em28xx *dev, u16 reg, u8 val,
>>>>  	int oldval;
>>>>  	u8 newval;
>>>>  
>>>> -	/* Uses cache for gpo/gpio registers */
>>>> -	if (reg == dev->reg_gpo_num)
>>>> -		oldval = dev->reg_gpo;
>>>> -	else if (reg == dev->reg_gpio_num)
>>>> -		oldval = dev->reg_gpio;
>>>> -	else
>>>> -		oldval = em28xx_read_reg(dev, reg);
>>>> -
>>>> +	oldval = em28xx_read_reg(dev, reg);
>>>>  	if (oldval < 0)
>>>>  		return oldval;
>>> That's plain wrong, as it will break GPIO input.
>>>
>>> With GPIO, you can write either 0 or 1 to a GPIO output port. So, your
>>> code works for output ports.
>>>
>>> However, an input port requires an specific value (either 1 or 0 depending
>>> on the GPIO circuitry). If the wrong value is written there, the input port
>>> will stop working.
>>>
>>> So, you can't simply read a value from a GPIO input and write it. You need
>>> to shadow the GPIO write values instead.
>> I don't understand what you mean.
>> Why can I not read the value of a GPIO input and write it ?
> Because, depending on the value you write, it can transform the input into an
> output port.

I don't get it.
We always write to the GPIO register. That's why these functions are
called em28xx_write_* ;)
Whether the write operation is sane or not (e.g. because it modifies the
bit corresponding to an input line) is not subject of these functions.


Frank