Erected Penis in porn movies

How penis is erected for so long in porn movies. There may be one of the below reason for that.

1. Male porn actors almost always do special exercises to maintain erection and stamina for a long time.
2. Viagra or similar drugs are used before shooting to keep maintain an erection regardless of the nervous status of the actor. Also, steroids (and lots of cardio) help them keep thrusting sans pause for inhumanly long.
3. Almost no porn videos are made in a single shot. There is time in between cuts for ejaculation, rest, consuming super-nutritious food and drinks and re-erection. It is also good for female actors to have some RnR in-between shots so that their vaginas do not get too sore. Sometimes, if the situation permits, and mostly in case of BDSM movies, one video is shot over days, even weeks. The editing makes it look like its all in an hour or so.
4. Permanent implants in the penis to make it appear hardened all the time. Also there are pacemaker-like electrical implants on the erection-related nerves or water-balloons replacing the spongy muscles to get hard-on-demand.