linux-kernel - Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs

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Message-ID: <C1886F9A-1799-4E3D-9153-579D31488695@zytor.com>
Date:   Thu, 11 Aug 2022 12:57:25 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Jonathan Corbet <corbet@....net>,
        Russell King <linux@...linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Arnd Bergmann <arnd@...db.de>
CC:     linux-gpio@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH] gpio: Allow user to customise maximum number of GPIOs

On August 9, 2022 3:40:38 AM PDT, Christophe Leroy <christophe.leroy@...roup.eu> wrote:
>At the time being, the default maximum number of GPIOs is set to 512
>and can only get customised via an architecture specific
>CONFIG_ARCH_NR_GPIO.
>
>The maximum number of GPIOs might be dependent on the number of
>interface boards and is somewhat independent of architecture.
>
>Allow the user to select that maximum number outside of any
>architecture configuration. To enable that, re-define a
>core CONFIG_ARCH_NR_GPIO for architectures which don't already
>define one. Guard it with a new hidden CONFIG_ARCH_HAS_NR_GPIO.
>
>Only two architectures will need CONFIG_ARCH_HAS_NR_GPIO: x86 and arm.
>
>On arm, do like x86 and set 512 as the default instead of 0, that
>allows simplifying the logic in asm-generic/gpio.h
>
>Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
>---
> Documentation/driver-api/gpio/legacy.rst |  2 +-
> arch/arm/Kconfig                         |  3 ++-
> arch/arm/include/asm/gpio.h              |  1 -
> arch/x86/Kconfig                         |  1 +
> drivers/gpio/Kconfig                     | 14 ++++++++++++++
> include/asm-generic/gpio.h               |  6 ------
> 6 files changed, 18 insertions(+), 9 deletions(-)
>
>diff --git a/Documentation/driver-api/gpio/legacy.rst b/Documentation/driver-api/gpio/legacy.rst
>index 9b12eeb89170..566b06a584cf 100644
>--- a/Documentation/driver-api/gpio/legacy.rst
>+++ b/Documentation/driver-api/gpio/legacy.rst
>@@ -558,7 +558,7 @@ Platform Support
> To force-enable this framework, a platform's Kconfig will "select" GPIOLIB,
> else it is up to the user to configure support for GPIO.
> 
>-It may also provide a custom value for ARCH_NR_GPIOS, so that it better
>+It may also provide a custom value for CONFIG_ARCH_NR_GPIO, so that it better
> reflects the number of GPIOs in actual use on that platform, without
> wasting static table space.  (It should count both built-in/SoC GPIOs and
> also ones on GPIO expanders.
>diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>index 53e6a1da9af5..e55b6560fe4f 100644
>--- a/arch/arm/Kconfig
>+++ b/arch/arm/Kconfig
>@@ -14,6 +14,7 @@ config ARM
> 	select ARCH_HAS_KCOV
> 	select ARCH_HAS_MEMBARRIER_SYNC_CORE
> 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>+	select ARCH_HAS_NR_GPIO
> 	select ARCH_HAS_PTE_SPECIAL if ARM_LPAE
> 	select ARCH_HAS_PHYS_TO_DMA
> 	select ARCH_HAS_SETUP_DMA_OPS
>@@ -1243,7 +1244,7 @@ config ARCH_NR_GPIO
> 	default 352 if ARCH_VT8500
> 	default 288 if ARCH_ROCKCHIP
> 	default 264 if MACH_H4700
>-	default 0
>+	default 512
> 	help
> 	  Maximum number of GPIOs in the system.
> 
>diff --git a/arch/arm/include/asm/gpio.h b/arch/arm/include/asm/gpio.h
>index f3bb8a2bf788..4ebbb58f06ea 100644
>--- a/arch/arm/include/asm/gpio.h
>+++ b/arch/arm/include/asm/gpio.h
>@@ -2,7 +2,6 @@
> #ifndef _ARCH_ARM_GPIO_H
> #define _ARCH_ARM_GPIO_H
> 
>-/* Note: this may rely upon the value of ARCH_NR_GPIOS set in mach/gpio.h */
> #include <asm-generic/gpio.h>
> 
> /* The trivial gpiolib dispatchers */
>diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>index f9920f1341c8..a8c956abc21e 100644
>--- a/arch/x86/Kconfig
>+++ b/arch/x86/Kconfig
>@@ -82,6 +82,7 @@ config X86
> 	select ARCH_HAS_MEM_ENCRYPT
> 	select ARCH_HAS_MEMBARRIER_SYNC_CORE
> 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>+	select ARCH_HAS_NR_GPIO
> 	select ARCH_HAS_PMEM_API		if X86_64
> 	select ARCH_HAS_PTE_DEVMAP		if X86_64
> 	select ARCH_HAS_PTE_SPECIAL
>diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
>index 0642f579196f..250b50ed44e1 100644
>--- a/drivers/gpio/Kconfig
>+++ b/drivers/gpio/Kconfig
>@@ -11,6 +11,9 @@ config ARCH_HAVE_CUSTOM_GPIO_H
> 	  overriding the default implementations.  New uses of this are
> 	  strongly discouraged.
> 
>+config ARCH_HAS_NR_GPIO
>+	bool
>+
> menuconfig GPIOLIB
> 	bool "GPIO Support"
> 	help
>@@ -22,6 +25,17 @@ menuconfig GPIOLIB
> 
> if GPIOLIB
> 
>+config ARCH_NR_GPIO
>+	int "Maximum number of GPIOs" if EXPERT
>+	depends on !ARCH_HAS_NR_GPIO
>+	default 512
>+	help
>+	  This allows the user to select the maximum number of GPIOs the
>+	  kernel must support. When the architecture defines a number
>+	  through CONFIG_ARCH_NR_GPIO, that value is taken and the user
>+	  cannot change it. Otherwise it defaults to 512 and the user
>+	  can change it when CONFIG_EXPERT is set.
>+
> config GPIOLIB_FASTPATH_LIMIT
> 	int "Maximum number of GPIOs for fast path"
> 	range 32 512
>diff --git a/include/asm-generic/gpio.h b/include/asm-generic/gpio.h
>index aea9aee1f3e9..ee090f534ab8 100644
>--- a/include/asm-generic/gpio.h
>+++ b/include/asm-generic/gpio.h
>@@ -24,13 +24,7 @@
>  * actually an estimate of a board-specific value.
>  */
> 
>-#ifndef ARCH_NR_GPIOS
>-#if defined(CONFIG_ARCH_NR_GPIO) && CONFIG_ARCH_NR_GPIO > 0
> #define ARCH_NR_GPIOS CONFIG_ARCH_NR_GPIO
>-#else
>-#define ARCH_NR_GPIOS		512
>-#endif
>-#endif
> 
> /*
>  * "valid" GPIO numbers are nonnegative and may be passed to

This seems very odd to me. GPIOs can be, and often are, attached to peripheral buses which means that the *same system* can have anything from none to thousands of gpios ..