drm/i915/display: Store pipe name in trace events

The first part[1] of the LWN series on using TRACE_EVENT() mentions
about TP_printk():

    "Do not create new tracepoint-specific helpers, because that will
    confuse user-space tools that know about the TRACE_EVENT() helper
    macros but will not know how to handle ones created for individual
    tracepoints."

It seems this is what we ended up doing when using pipe_name() in
TP_printk().

For example, the format for the intel_pipe_update_start event is as
follows:

    # cat /sys/kernel/debug/tracing/events/i915/intel_pipe_update_start/format
    name: intel_pipe_update_start
    ID: 1136
    format:
            field:unsigned short common_type;       offset:0;       size:2; signed:0;
            field:unsigned char common_flags;       offset:2;       size:1; signed:0;
            field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
            field:int common_pid;   offset:4;       size:4; signed:1;

            field:__data_loc char[] dev;    offset:8;       size:4; signed:0;
            field:enum pipe pipe;   offset:12;      size:4; signed:1;
            field:u32 frame;        offset:16;      size:4; signed:0;
            field:u32 scanline;     offset:20;      size:4; signed:0;
            field:u32 min;  offset:24;      size:4; signed:0;
            field:u32 max;  offset:28;      size:4; signed:0;

    print fmt: "dev %s, pipe %c, frame=%u, scanline=%u, min=%u, max=%u", __get_str(dev), ((REC->pipe) + 'A'), REC->frame, REC->scanline, REC->min, REC->max

The call to pipe_name(__entry->pipe) is expanted to ((REC->pipe) + 'A')
and that's how the format is saved.

Even though the output from /sys/kernel/debug/tracing/trace will look
correct (because it is generated in the kernel), we will see corrupted
lines when using a tool like trace-cmd to view the data.

While the output looks correct when looking at
/sys/kernel/debug/tracing/trace, we see corrupted lines when viewing the
trace data with "trace-cmd report":

    $ trace-cmd report \
    > | sed -n 's/.*dev 0000:00:02\.0, \(pipe .\).*/\1/p' \
    > | cat -v | uniq -c
         34 pipe ^A

, where ^A is a non-printable character.

As a fix, let's store the pipe name directly in the event. The fix was
done by applying the following sed script:

    s/__field\s*(\s*enum\s\+pipe\s*,\s*pipe\s*)/__field(char, pipe_name)/
    s/__entry\s*->\s*pipe\s*=\s*\([^;]\+\);/__entry->pipe_name = pipe_name(\1);/
    s/pipe_name\s*(\s*__entry\s*->\s*pipe\s*)/__entry->pipe_name/

After these changes, using the same example, we have the following:

    $ trace-cmd report \
    > | sed -n 's/.*dev 0000:00:02\.0, \(pipe .\).*/\1/p' \
    > | cat -v | sort | uniq -c
        396 pipe A
         34 pipe B

[1] https://lwn.net/Articles/379903/

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016135300.21428-4-gustavo.sousa@intel.com

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