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

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

Am 13.04.2013 20:08, schrieb Mauro Carvalho Chehab:
> Em Sat, 13 Apr 2013 19:46:20 +0200
> Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:
>
>> 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.
> Writing is sane: GPIO input lines requires writing as well, in order to 
> set it to either pull-up or pull-down mode (not sure if em28xx supports
> both ways).
>
> So, the driver needs to know if it will write there a 0 or 1, and this is part
> of its GPIO configuration.
>
> Let's assume that, on a certain device, you need to write "1" to enable that
> input.
>
> A read I/O to that port can return either 0 or 1. 
>
> Giving an hypothetical example, please assume this code:
>
> static int write_gpio_bits(u32 out, u32 mask)
> {
> 	u32 gpio = (read_gpio_ports() & ~mask) | (out & mask);
> 	write_gpio_ports(gpio);
> }
>
>
> ...
> 	/* Use bit 1 as input GPIO */
> 	write_gpio_bits(1, 1);
>
> 	/* send a reset via bit 2 GPIO */
> 	write_gpio_bits(2, 2);
> 	write_gpio_bits(0, 2);
> 	write_gpio_bits(2, 2);
>
> If, at the time the above code runs, the input bit 1 is at "0" state,
> the subsequent calls will disable the input.
>
> If, instead, only the write operations are cached like:
>
> static int write_gpio_bits(u32 out, u32 mask)
> {
> 	static u32 shadow_cache;
>
> 	shadow_cache = (shadow_cache & ~mask) | (out & mask);
> 	write_gpio_ports(gpio);
> }
>
> there's no such risk, as it will keep using "1" for the input bit.

Hmm... ok, now I understand what you mean.
Are you sure the Empia chips are really working this way ?
I checked the em25xx datasheet (excerpt) and it talks about separate
registers for GPIO configuration (unfortunately without explaining their
function in detail).
I going to do some tests with the Laplace webcam, so far it seems to be
working fine without this caching stuff.
But the reverse-engineering possibilities are quite limited, so someone
with a detailed datasheet should really look this up.

Regards,
Frank