clear_user函数被用于将用户空间的内存块清零。
The clear_user function is used to zero a block of memory in user space. This function takes a pointer in user space and a size to zero, which is defined in bytes.
滚屏计数值的大小将递增，直到超过一块图素的宽度时再被清零。
The scroller location is incremented and zeroed again if it exceeds the width of a tile.
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