Date: Fri, 13 Apr 2018 14:10:22 -0700 From: Laura Abbott <labbott@...hat.com> To: Phil Reid <preid@...ctromag.com.au>, Linus Walleij <linus.walleij@...aro.org> Cc: Kees Cook <keescook@...omium.org>, Lukas Wunner <lukas@...ner.de>, Rasmus Villemoes <linux@...musvillemoes.dk>, "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, kernel-hardening@...ts.openwall.com Subject: Re: [PATCHv4] gpio: Remove VLA from gpiolib On 04/12/2018 05:39 PM, Phil Reid wrote: > On 12/04/2018 16:38, Linus Walleij wrote: >> On Wed, Apr 11, 2018 at 3:03 AM, Laura Abbott <labbott@...hat.com> wrote: >> >>> The new challenge is to remove VLAs from the kernel >>> (see https://lkml.org/lkml/2018/3/7/621) to eventually >>> turn on -Wvla. >>> >>> Using a kmalloc array is the easy way to fix this but kmalloc is still >>> more expensive than stack allocation. Introduce a fast path with a >>> fixed size stack array to cover most chip with gpios below some fixed >>> amount. The slow path dynamically allocates an array to cover those >>> chips with a large number of gpios. >>> >>> Reviewed-and-tested-by: Lukas Wunner <lukas@...ner.de> >>> Signed-off-by: Lukas Wunner <lukas@...ner.de> >>> Signed-off-by: Laura Abbott <labbott@...hat.com> >>> --- >>> v4: Changed some local variables to avoid coccinelle warnings. Added a >>> warning if the number of GPIOs exceeds the current fast path define. >>> >>> Lukas, I kept your Tested-by because the changes were pretty minimal. >>> Let me know if you want to run the tests again. >> >> This patch is starting to look really good. >> >>> +/* >>> + * Number of GPIOs to use for the fast path in set array >>> + */ >>> +#define FASTPATH_NGPIO 256 >> >> There is still some comment about this. >> >> And now that I am also tryint to think I wonder about it, we >> have a global ARCH_NR_GPIOS that is typically 512. >> Some archs set it up. >> >> This define is something of an abomination, in the ARM >> case it comes from arch/arm/include/asm/gpio.h >> where #define ARCH_NR_GPIOS CONFIG_ARCH_NR_GPIO >> where the latter is a Kconfig option that is mostly 512 for >> most ARM systems. >> >> Well, ARM looks like this: >> >> config ARCH_NR_GPIO >> int >> default 2048 if ARCH_SOCFPGA >> default 1024 if ARCH_BRCMSTB || ARCH_SHMOBILE || ARCH_TEGRA || \ >> ARCH_ZYNQ >> default 512 if ARCH_EXYNOS || ARCH_KEYSTONE || SOC_OMAP5 || \ >> SOC_DRA7XX || ARCH_S3C24XX || ARCH_S3C64XX || ARCH_S5PV210 >> default 416 if ARCH_SUNXI >> default 392 if ARCH_U8500 >> default 352 if ARCH_VT8500 >> default 288 if ARCH_ROCKCHIP >> default 264 if MACH_H4700 >> default 0 >> help >> Maximum number of GPIOs in the system. >> >> If unsure, leave the default value. >> >> So if FASTPATH_NGPIO should be anything else than >> ARCH_NR_GPIO this has to be established somewhere >> as a floor or half or something, but I would just set it as >> the same as ARCH_NR_GPIOS... >> >> The main reason this define exist is for this function >> from <linux/gpio/consumer.h>: >> >> /* Convert between the old gpio_ and new gpiod_ interfaces */ >> struct gpio_desc *gpio_to_desc(unsigned gpio); >> >> Nowadays that fact is a bit obscured since the variable is >> only used when assigning the base (in the global GPIO >> number space, which is what we want to get rid of but >> sigh) in gpiochip_find_base() where it attempts to place >> a newly allocated gpiochip in the higher region of this >> numberspace since the embedded SoC GPIO base tends >> to be 0, on old platforms. >> >> So I don't know about this. >> >> Can't we just use ARCH_NR_GPIOS? >> >> Very few systems have more than 512 assigned global >> GPIO numbers and those are FPGA experimental machines. >> >> In the long run obviously I want to get rid of these defines >> altogether and only allocate GPIO descriptos dynamically >> so as you see I am reluctant to add new numberspace weirdness >> around here. > Isn't that for total GPIO's in the system? > And the arrays just need to cater for max per chip? > From what I can understand of the code which is admittedly limited. > > Yeah the switch back to 256 was a mistake on my end (I think I grabbed an incorrect version for my base). ARCH_NR_GPIOs is the total number in the system which may be multiple chips so yes we would be possibly allocating more space than necessary. unsigned long fastpath[2 * BITS_TO_LONGS(FASTPATH_NGPIO)] unsigned long fastpath[2 * BITS_TO_LONGS(512)] unsigned long fastpath[2 * DIV_ROUND_UP(512, 8 * sizeof(long))] so we end up with 128 bytes on the stack total assuming I can do math correctly. I think this a fairly reasonable amount though, even if we are over-estimating if there are multiple chips. Thanks, Laura
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