Reading the EGL delivery trace (`BLINKGTK_GX_LOG`)

Start with BLINKGTK_GX_LOG=1 and BlinkGTK writes one line per event for the
EGL delivery path. Use it to see how far a frame got when nothing appears on
screen, or when presentation is slow.

BJGX <event> t=<ms since start> buf=<wl_buffer wire id> submit=<serial> …

buf= is the wire id of the wl_buffer on the compositor side
submit= is a serial we assign per commit; the pair (buf=, submit=)
identifies one submission
submit= is not always a number. Lines where no serial was assigned carry
submit=-. Parsing it as an integer will stop on those lines
t= is BlinkGTK's own elapsed milliseconds, not the compositor clock

Events and fields

Event	Fields on the line
`bind`	`t=` `iface=` `name=`
`clock`	`t=` `clk_id=`
`commit`	`t=` `buf=` `submit=` `op=` `w=` `h=` `stride=` `fmt=` `mod=`
	`t=` `buf=` `submit=` `op=` `w=` `h=` `src=`
`discard`	`t=` `buf=` `submit=`
`present`	`t=` `buf=` `submit=` `tv=` `refresh=` `msc=` `flags=` `flagnames=` `clk=`
`probe`	`t=` `buf=` `submit=` `result=` `w=` `h=` `stride=`
`release`	`t=` `buf=` `submit=`
`skip`	`t=` `buf=` `submit=` `reason=`

What each event means

Event	When it appears
`commit`	A buffer was attached to the surface and committed
`present`	The compositor reported that it was displayed
`discard`	The compositor reported that it was not displayed
`release`	The compositor returned the buffer (safe to reuse)
`skip`	Nothing was sent. `reason=` says why
`probe`	Content inspection result. `result=` carries the verdict
`bind`	A Wayland interface was bound
`clock`	Clock id used for presentation timestamps

present and discard are mutually exclusive — exactly one of them arrives
for a given content update.

Things that trip parsers

op= appears only on commit lines. It is not present on present lines.
What present carries instead is clk= (below)
op= takes one of attach+viewport, attach, destroy, none
The tail of a commit line depends on the path: the dmabuf path carries
stride= fmt= mod=, while the software (shm) path carries src=shm and omits
stride=, fmt= and mod=
clk= tells you which clock the tv= timestamp on a present line is based
on — a POSIX clockid_t (1 = CLOCK_MONOTONIC, 4 = CLOCK_MONOTONIC_RAW).
It is - when the compositor did not report one

Deciding whether content reached the screen

Look for present lines.

content_present (probe#N=YES/BLANK) judges by reading buffer contents from
the CPU side. On some systems that read is not possible at all (virtualised
GPUs, for example). There it stays UNKNOWN, so using it as the criterion reads
as "nothing was displayed" when in fact everything was.

present comes from the compositor, so it holds on those systems too.

This table is generated from the implementation