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Date:   Tue, 14 Apr 2020 21:57:15 +0200
From:   Michael Walle <michael@walle.cc>
To:     Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc:     linux-gpio <linux-gpio@vger.kernel.org>,
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Subject: Re: [PATCH v2 10/16] gpio: add a reusable generic gpio_chip using
 regmap
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Hi Mark, Hi Bartosz, Hi Linus,

Am 2020-04-14 20:41, schrieb Michael Walle:
> Am 2020-04-14 19:00, schrieb Bartosz Golaszewski:
>> wt., 14 kwi 2020 o 12:07 Michael Walle <michael@walle.cc> napisał(a):
>>> >>
>>> >> So the best from a user perspective I've could come up with was:
>>> >>
>>> >>    ->base_reg = GPIO_REGMAP_ADDR(addr);
>>> >>
>>> >> I'm open for suggestions.
>>> >>
>>> >
>>> > Maybe setting the pointer to ERR_PTR(-ENOENT) which will result in
>>> > IS_ERR() returning true?
>>> 
>>> Unfortunatly, its not a pointer, but only a regular unsigned int (ie
>>> the type the regmap API has for its "reg" property). It could be a
>>> pointer of course but then the user would have to allocate additional
>>> memory.
>>> 
>>> -michael
>>> 
>> 
>> Eek, of course it's not a pointer. If possible I'd like to avoid this
>> GPIO_REGMAP_ADDR() macro, so how about having some separate field for
>> invalid offsets making every offset 'valid' by default?
> 
> IMHO this has the same problems as mentioned in the response to Mark's
> idea. Normally, the user sets only some addresses, thus he has to mark
> all other as invalid. And if you add another address, you have to touch
> all the drivers to mark it as invalid.
> 
> We could add some force bits like the "use_ack" flag in the bgpio 
> driver,
> where you can force the use of the value 0. But I'd really like to find
> a better way..

So what about the following:

#define GPIO_REGMAP_ADDR_ZERO (unsigned int)(-1)

So this way the user might assign the base addresses the normal way
except when he wants to use zero, in that case he has to use

   ->base_adr = GPIO_REGMAP_ADDR_ZERO;

gpio-regmap.c could use then:

if (base_addr)
   something_useful(gpio_regmap_addr(base_addr));

unsigned int gpio_regmap_addr(unsigned int addr)
{
   return (addr == GPIO_REGMAP_ADDR_ZERO) ? 0 : addr;
}

-michael