LOWER_32_BITS(9) Driver Basics LOWER_32_BITS(9)NAME
lower_32_bits - return bits 0-31 of a number
SYNOPSIS
lower_32_bits(n);
ARGUMENTS
n
the number we're accessing
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 LOWER_32_BITS(9)
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TRACE_PRINTK(9) Driver Basics TRACE_PRINTK(9)NAME
trace_printk - printf formatting in the ftrace buffer
SYNOPSIS
trace_printk(fmt, ...);
ARGUMENTS
fmt
the printf format for printing
...
variable arguments
NOTE
__trace_printk is an internal function for trace_printk and the ip is passed in via the trace_printk macro.
This function allows a kernel developer to debug fast path sections that printk is not appropriate for. By scattering in various printk
like tracing in the code, a developer can quickly see where problems are occurring.
This is intended as a debugging tool for the developer only. Please refrain from leaving trace_printks scattered around in your code.
(Extra memory is used for special buffers that are allocated when trace_printk is used)
A little optization trick is done here. If there's only one argument, there's no need to scan the string for printf formats. The trace_puts
will suffice. But how can we take advantage of using trace_puts when trace_printk has only one argument? By stringifying the args and
checking the size we can tell whether or not there are args. __stringify((__VA_ARGS__)) will turn into "() " with a size of 3 when there
are no args, anything else will be bigger. All we need to do is define a string to this, and then take its size and compare to 3. If it's
bigger, use do_trace_printk otherwise, optimize it to trace_puts. Then just let gcc optimize the rest.
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 TRACE_PRINTK(9)