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SubjectRe: [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@huawei.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

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