3d8b56d7b1
Update to commit d1ce350015 Documentation: ("Add
io_ordering.rst to driver-api manual").
Move ../zh_CN/io_ordering.txt to ../zh_CN/driver-api/io_ordering.rst.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Wu XiangCheng <bobwxc@email.cn>
Link: https://lore.kernel.org/r/c66f6d17c509c2c93f2afd30223c4bcf734f8317.1661431365.git.siyanteng@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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.. SPDX-License-Identifier: GPL-2.0
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.. include:: ../disclaimer-zh_CN.rst
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:Original: Documentation/driver-api/io_ordering.rst
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:翻译:
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林永听 Lin Yongting <linyongting@gmail.com>
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司延腾 Yanteng Si <siyanteng@loongson.cn>
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:校译:
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===========================
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对内存映射地址的I/O写入排序
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===========================
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在某些平台上，所谓的内存映射I/O是弱顺序。在这些平台上，驱动开发者有责任
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保证I/O内存映射地址的写操作按程序图意的顺序达到设备。通常读取一个“安全”
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设备寄存器或桥寄存器，触发IO芯片清刷未处理的写操作到达设备后才处理读操作，
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而达到保证目的。驱动程序通常在spinlock保护的临界区退出之前使用这种技术。
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这也可以保证后面的写操作只在前面的写操作之后到达设备（这非常类似于内存
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屏障操作，mb()，不过仅适用于I/O）。
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假设一个设备驱动程的具体例子::
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...
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CPU A: spin_lock_irqsave(&dev_lock, flags)
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CPU A: val = readl(my_status);
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CPU A: ...
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CPU A: writel(newval, ring_ptr);
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CPU A: spin_unlock_irqrestore(&dev_lock, flags)
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...
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CPU B: spin_lock_irqsave(&dev_lock, flags)
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CPU B: val = readl(my_status);
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CPU B: ...
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CPU B: writel(newval2, ring_ptr);
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CPU B: spin_unlock_irqrestore(&dev_lock, flags)
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...
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上述例子中，设备可能会先接收到newval2的值，然后接收到newval的值，问题就
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发生了。不过很容易通过下面方法来修复::
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...
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CPU A: spin_lock_irqsave(&dev_lock, flags)
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CPU A: val = readl(my_status);
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CPU A: ...
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CPU A: writel(newval, ring_ptr);
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CPU A: (void)readl(safe_register); /* 配置寄存器？*/
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CPU A: spin_unlock_irqrestore(&dev_lock, flags)
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...
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CPU B: spin_lock_irqsave(&dev_lock, flags)
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CPU B: val = readl(my_status);
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CPU B: ...
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CPU B: writel(newval2, ring_ptr);
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CPU B: (void)readl(safe_register); /* 配置寄存器？*/
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CPU B: spin_unlock_irqrestore(&dev_lock, flags)
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在解决方案中，读取safe_register寄存器，触发IO芯片清刷未处理的写操作，
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再处理后面的读操作，防止引发数据不一致问题。
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