Re: [PATCHv2 1/2] add basic register-field manipulation macros
From: Arend van Spriel <arend.vanspriel@broadcom.com>
Date: 2016-06-14 18:53:33
Also in:
lkml, netdev
On 14-06-16 13:44, Jakub Kicinski wrote:
C bitfields are problematic and best avoided. Developers interacting with hardware registers find themselves searching for easy-to-use alternatives. Common approach is to define structures or sets of macros containing mask and shift pair. Operations on the register are then performed as follows:
[...]
quoted hunk ↗ jump to hunk
Compared to Felix Fietkau's implementation from mt76 this one uses standard Linux and GCC functions such as is_power_of_2() and __builtin_ffsll(). Signed-off-by: Jakub Kicinski <redacted> --- v2: - change Felix's email address. include/linux/bitfield.h | 58 ++++++++++++++++++++++++++++++++++++++++++++++++ include/linux/log2.h | 6 +++++ 2 files changed, 64 insertions(+) create mode 100644 include/linux/bitfield.hdiff --git a/include/linux/bitfield.h b/include/linux/bitfield.h new file mode 100644 index 000000000000..9560d1877cbc --- /dev/null +++ b/include/linux/bitfield.h@@ -0,0 +1,58 @@ +/* + * Copyright (C) 2014 Felix Fietkau <nbd@nbd.name> + * Copyright (C) 2004 - 2009 Ivo van Doorn <IvDoorn@gmail.com> + * + * This program is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License version 2 + * as published by the Free Software Foundation + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + */ + +#ifndef _LINUX_BITFIELD_H +#define _LINUX_BITFIELD_H + +#include <asm/types.h> +#include <linux/bug.h> +#include <linux/log2.h> + +#define _bf_shf(x) (__builtin_ffsll(x) - 1) + +#define _BF_FIELD_CHECK(_mask, _val) \ + ({ \ + const u64 hi = (_mask) + (1ULL << _bf_shf(_mask)); \ + \ + BUILD_BUG_ON(!(_mask) || (hi && !is_power_of_2_u64(hi))); \ + BUILD_BUG_ON(__builtin_constant_p(_val) ? \ + ~((_mask) >> _bf_shf(_mask)) & (_val) : \ + 0); \ + })
I am sceptic whether it is useful to have 64-bit used here and there is a price to pay on (many) 32-bit architectures for using 64-bit operations. Maybe it is not an issue because it is inside BUILD_BUG_ON() macro.
+#define FIELD_PUT(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, _val); \
+ ((u32)(_val) << _bf_shf(_mask)) & (_mask); \
+ })
+
+#define FIELD_GET(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, 0); \
+ (u32)(((_val) & (_mask)) >> _bf_shf(_mask)); \
+ })
+
+#define FIELD_PUT64(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, _val); \
+ ((u64)(_val) << _bf_shf(_mask)) & (_mask); \
+ })
+
+#define FIELD_GET64(_mask, _val) \
+ ({ \
+ _BF_FIELD_CHECK(_mask, 0); \
+ (u64)(((_val) & (_mask)) >> _bf_shf(_mask)); \
+ })Is there really hardware out there that exposes 64-bit wide hardware registers? Regards, Arend