Re: [PATCH v1 4/8] gpiolib: Get rid of ARCH_NR_GPIOS

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Le 31/08/2022 à 22:55, Arnd Bergmann a écrit :
> On Wed, Aug 31, 2022, at 7:49 AM, Christophe Leroy wrote:
>> Le 30/08/2022 à 22:18, Andy Shevchenko a écrit :
>>> On Mon, Aug 29, 2022 at 7:19 PM Christophe Leroy
>>> <christophe.leroy@xxxxxxxxxx> wrote:
>>>>
>>>> Since commit 14e85c0e69d5 ("gpio: remove gpio_descs global array")
>>>> there is no limitation on the number of GPIOs that can be allocated
>>>> in the system since the allocation is fully dynamic.
>>>>
>>>> ARCH_NR_GPIOS is today only used in order to provide downwards
>>>> gpiobase allocation from that value, while static allocation is
>>>> performed upwards from 0. However that has the disadvantage of
>>>> limiting the number of GPIOs that can be registered in the system.
>>>>
>>>> To overcome this limitation without requiring each and every
>>>> platform to provide its 'best-guess' maximum number, rework the
>>>> allocation to allocate upwards, allowing approx 2 millions of
>>>> GPIOs.
>>>>
>>>> In order to still allow static allocation for legacy drivers, define
>>>> GPIO_DYNAMIC_BASE with the value 256 as the start for dynamic
>>>> allocation.
>>>
>>> Not sure about 256, but I understand that this can only be the best guess.
>>>
>>
>> Well, it's already just a precaution. Linus W's expectation is that
>> static ones are allocated at first, they should already be allocated
>> when we start doing dynamic allocations so he was even thinking that we
>> could have started at 0 already.
>>
>> But I can start higher if you think it is safer, maybe at 512 which is
>> the default ARCH_NR_GPIOS today.
> 
> FWIW, I went through the drivers that set the base to a value other than
> zero or -1, to see what they use:
> 

> arch/arm/mach-s3c/gpio-samsung.c:			.base	= S3C2410_GPM(0), // 384

> arch/arm/mach-s3c/mach-h1940.c:	.base			= H1940_LATCH_GPIO(0), // 384 + 22

> drivers/mfd/asic3.c:	asic->gpio.base = pdata->gpio_base; // 300 + 100
> drivers/mfd/htc-i2cpld.c:	gpio_chip->base            = plat_chip_data->gpio_in_base; // 192 + 16 + 8*8


Ok, so 256 is not enough, have to go to 512.

Christophe