[PATCHv7] rtc: Add support for Intersil ISL12057 I2C RTC chip

Andrew Morton
2013-12-19T22:07:26Z

On Thu, 19 Dec 2013 22:53:16 +0100 Arnaud Ebalard <arno@natisbad.org> wrote:

> +static int isl12057_i2c_validate_chip(struct regmap *regmap)
> +{
> + u8 regs[ISL12057_MEM_MAP_LEN];
> + u8 mask[ISL12057_MEM_MAP_LEN] = { 0x80, 0x80, 0x80, 0xf8,
> + 0xc0, 0x60, 0x00, 0x00,
> + 0x00, 0x00, 0x00, 0x00,
> + 0x00, 0x00, 0x60, 0x7c };
> + int ret, i;
> +
> + ret = regmap_bulk_read(regmap, 0, regs, ISL12057_MEM_MAP_LEN);
> + if (ret)
> + return ret;
> +
> + for (i = 0; i < ISL12057_MEM_MAP_LEN; ++i) {
> + if (regs[i] & mask[i]) /* check if bits are cleared */
> + return -ENODEV;
> + }
> +
> + return 0;
> +}

When I build this for x86_64, mask[] is assembled on-stack at runtime:

leaq -48(%rbp), %r14 #, tmp130
xorl %esi, %esi #
movl $16, %ecx #,
movq %rax, %rdi # regmap,
incq .LPBX1+552(%rip) # *.LPBX1
movb $-128, -64(%rbp) #, mask
movq %r14, %rdx # tmp130,
movb $-128, -63(%rbp) #, mask
movb $-128, -62(%rbp) #, mask
movb $-8, -61(%rbp) #, mask
movb $-64, -60(%rbp) #, mask
movb $96, -59(%rbp) #, mask
movb $0, -58(%rbp) #, mask
movb $0, -57(%rbp) #, mask
movb $0, -56(%rbp) #, mask
movb $0, -55(%rbp) #, mask
movb $0, -54(%rbp) #, mask
movb $0, -53(%rbp) #, mask
movb $0, -52(%rbp) #, mask
movb $0, -51(%rbp) #, mask
movb $96, -50(%rbp) #, mask
movb $124, -49(%rbp) #, mask
call regmap_bulk_read #

If we make `mask' static const, that all gets done at compile time instead:

leaq -48(%rbp), %r14 #, tmp134
xorl %esi, %esi #
movl $16, %ecx #,
movq %rax, %rdi # regmap,
incq .LPBX1+552(%rip) # *.LPBX1
movq %r14, %rdx # tmp134,
call regmap_bulk_read #

much nicer.