/*
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* Copyright (C) 2013 Philippe Gerum <rpm@xenomai.org>.
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
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*/
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#ifndef _COBALT_UAPI_KERNEL_URW_H
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#define _COBALT_UAPI_KERNEL_URW_H
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#include <linux/types.h>
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/*
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* A restricted version of the kernel seqlocks with a slightly
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* different interface, allowing for unsynced reads with concurrent
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* write detection, without serializing writers. Caller should
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* provide for proper locking to deal with concurrent updates.
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*
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* urw_t lock = URW_INITIALIZER;
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* urwstate_t tmp;
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*
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* unsynced_read_block(&tmp, &lock) {
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* (will redo until clean read)...
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* }
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*
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* unsynced_write_block(&tmp, &lock) {
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* ...
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* }
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*
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* This code was inspired by Wolfgang Mauerer's linux/seqlock.h
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* adaptation for Xenomai 2.6 to support the VDSO feature.
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*/
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typedef struct {
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__u32 sequence;
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} urw_t;
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typedef struct {
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__u32 token;
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__u32 dirty;
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} urwstate_t;
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#define URW_INITIALIZER { 0 }
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#define DEFINE_URW(__name) urw_t __name = URW_INITIALIZER
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#ifndef READ_ONCE
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#define READ_ONCE ACCESS_ONCE
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#endif
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static inline void __try_read_start(const urw_t *urw, urwstate_t *tmp)
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{
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__u32 token;
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repeat:
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token = READ_ONCE(urw->sequence);
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smp_rmb();
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if (token & 1) {
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cpu_relax();
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goto repeat;
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}
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tmp->token = token;
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tmp->dirty = 1;
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}
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static inline void __try_read_end(const urw_t *urw, urwstate_t *tmp)
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{
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smp_rmb();
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if (urw->sequence != tmp->token) {
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__try_read_start(urw, tmp);
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return;
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}
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tmp->dirty = 0;
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}
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static inline void __do_write_start(urw_t *urw, urwstate_t *tmp)
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{
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urw->sequence++;
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tmp->dirty = 1;
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smp_wmb();
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}
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static inline void __do_write_end(urw_t *urw, urwstate_t *tmp)
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{
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smp_wmb();
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tmp->dirty = 0;
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urw->sequence++;
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}
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static inline void unsynced_rw_init(urw_t *urw)
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{
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urw->sequence = 0;
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}
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#define unsynced_read_block(__tmp, __urw) \
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for (__try_read_start(__urw, __tmp); \
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(__tmp)->dirty; __try_read_end(__urw, __tmp))
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#define unsynced_write_block(__tmp, __urw) \
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for (__do_write_start(__urw, __tmp); \
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(__tmp)->dirty; __do_write_end(__urw, __tmp))
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#endif /* !_COBALT_UAPI_KERNEL_URW_H */
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