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    SubjectRe: [Patch] Tracing/ftrace: Adds a marker to allow user comments

    is there a logic behind trace_seq_print_cont() printing the terminating
    newline only, when there actually is no TRACE_CONT entry?

    Hmm, wait a minute, I don't understand how this thing works at all.

    Let's take for instance print_lat_fmt() which is the first user of
    trace_seq_print_cont(). Now, print_lat_fmt() does

    struct trace_entry *entry = iter->ent;

    and uses 'entry'. Let's assume it is of type TRACE_PRINT. It does

    case TRACE_PRINT:
    seq_print_ip_sym(s, field->print.ip, sym_flags);
    trace_seq_printf(s, ": %s", field->print.buf);
    if (field->flags & TRACE_FLAG_CONT)
    trace_seq_print_cont(s, iter);

    Ok, it prints the beginning of the message, and if there should be
    continuation blocks, it calls trace_seq_print_cont(), defined as

    static void
    trace_seq_print_cont(struct trace_seq *s, struct trace_iterator *iter)
    struct trace_array *tr = iter->tr;
    struct trace_array_cpu *data = tr->data[iter->cpu];
    struct trace_entry *ent;

    ent = trace_entry_idx(tr, data, iter, iter->cpu);
    if (!ent || ent->type != TRACE_CONT) {
    trace_seq_putc(s, '\n');

    do {
    trace_seq_printf(s, "%s", ent->cont.buf);
    __trace_iterator_increment(iter, iter->cpu);
    ent = trace_entry_idx(tr, data, iter, iter->cpu);
    } while (ent && ent->type == TRACE_CONT);

    Here it uses trace_entry_idx() to get 'ent'. What's the difference to
    iter->ent? I don't understand how trace_entry_idx() works, but looking
    at how it is used, it must return the pointer to the *next* entry in
    the ring buffer. So I don't understand the name of the function, and I
    don't see a call to __trace_iterator_increment(), which is confusing.

    If contrary to the assumption, 'ent' is not a continuation, it prints
    the terminating newline. This is an exceptional case, as the original
    entry was marked as having continuation entries.

    The normal case then is to execute the do-while, until it hits a
    non-continuation entry. Here it does *not* print the terminating newline.

    Steven, could you explain what is going on here?


    On Thu, 4 Sep 2008 20:30:58 +0300
    Pekka Paalanen <> wrote:

    > b) have a customisable trace_seq_print_cont()
    > I think I'd favour b). When I tried to use trace_seq_print_cont(),
    > I found it difficult to control the printing of \n at the end of
    > message. So I would implement my own routine to handle also what
    > trace_seq_print_cont() does, filtering the text while copying it
    > to struct trace_seq. I'm not sure where this function should live.
    > It could be useful to all tracers, that want "one line per marker
    > entry" style output, so it could live in trace.c.

    Pekka Paalanen

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