linux-kernel - Re: [RFC V2 PATCH 05/11] cxl/mem: Trace General Media Event Record

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Date:   Mon, 17 Oct 2022 13:21:43 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Ben Widawsky" <bwidawsk@...nel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>
Subject: Re: [RFC V2 PATCH 05/11] cxl/mem: Trace General Media Event Record

On Mon, 17 Oct 2022 17:37:17 +0100
Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:

> Looking at other similar cases though and we have a lot of use
> of trace_seq_printf() e.g. libata_trace_parse_status() though note
> there is some magic macro stuff in include/trace/events/libata.h 
> to tie that together.
> https://elixir.bootlin.com/linux/latest/source/drivers/ata/libata-trace.c#L14
> 
> That seems to get you access to the actual buffer we are printing into
> in similar cases.

Looking at the code you linked to, I wonder why __print_flags() wasn't used?

For instance, you have:

const char *
libata_trace_parse_status(struct trace_seq *p, unsigned char status)
{
        const char *ret = trace_seq_buffer_ptr(p);

        trace_seq_printf(p, "{ ");
        if (status & ATA_BUSY)
                trace_seq_printf(p, "BUSY ");
        if (status & ATA_DRDY)
                trace_seq_printf(p, "DRDY ");
        if (status & ATA_DF)
                trace_seq_printf(p, "DF ");
        if (status & ATA_DSC)
                trace_seq_printf(p, "DSC ");
        if (status & ATA_DRQ)
                trace_seq_printf(p, "DRQ ");
        if (status & ATA_CORR)
                trace_seq_printf(p, "CORR ");
        if (status & ATA_SENSE)
                trace_seq_printf(p, "SENSE ");
        if (status & ATA_ERR)
                trace_seq_printf(p, "ERR ");
        trace_seq_putc(p, '}');
        trace_seq_putc(p, 0);

        return ret;
}


Which is just a re-implementation of:

__print_flags(status, " ", 
	{ ATA_BUSY, "BUSY" },
	{ ATA_DRDY, "DRDY" },
	{ ATA_DF, "DF" },
	{ ATA_DSC, "DSC" },
	{ ATA_DRQ, "DRQ" },
	{ ATA_CORR, "CORR" },
	{ ATA_SENSE, "SENSE" },
	{ ATA_ERR, "ERR" })


The major difference between the two, is that libtraceevent will be able to
parse the above and convert the status bits into strings, whereas using
libata_trace_parse_status() will just give you a parsing error.

That is, perf and trace-cmd will not be able to parse it unless you write a
separate plugin for libtraceevent to do it but that means you'll have
duplicate code.

I know you just want echo and cat, but that will still work, and this will
make it work for the tooling as well.

-- Steve