Add some more documentation for devtools and OHOS debugging. (#83)

* Add some more documentation for devtools and OHOS debugging.

Signed-off-by: Narfinger <[email protected]>

* Incorporate suggestions.

Co-authored-by: Josh Matthews <[email protected]>
Signed-off-by: Narfinger <[email protected]>

---------

Signed-off-by: Narfinger <[email protected]>
Co-authored-by: Josh Matthews <[email protected]>

Author: Narfinger

src/hacking/debugging-on-openharmony.md

@@ -9,11 +9,12 @@ When debugging, it can often be useful to start servo from the commandline inste
 
 ```shell
 # Run this command to see which parameters can be passed to an ohos app
-hdc shell aa start --help 
+hdc shell aa start --help
 # Start servo from the commandline
 hdc shell aa start -a EntryAbility -b org.servo.servo
 # --ps=<arg> <value> can be used to pass arguments with values to servoshell.
-# The space between arg and value is mandatory when using `--ps`.
+# The space between arg and value is mandatory when using `--ps`, meaning `--ps=--log-filter warn` is
+# correct while `--ps=--log-filter=warn` is not.
 # For pure flags without a value use `--psn <flag>`
 hdc shell aa start -a EntryAbility -b org.servo.servo --ps=--log-filter "warn"
 # Use `-U <url>` to let servo load a custom URL.
@@ -41,18 +42,18 @@ hdc shell hilog --domain=0xE0C3 --level=ERROR
 
 ### Log level
 
-Whether log statements are visible depends on multiple conditions: 
+Whether log statements are visible depends on multiple conditions:
 
 1. The compile-time max log level of the `log` crate. See [log compile-time filters].
   Note: The `release_max_level_<level>` features of the log crate check if `debug_assertions = false` is set
-  to determine the release filters should apply or not. 
+  to determine the release filters should apply or not.
 2. The runtime `log` global filter per module filters set in servoshell. Since environment variables aren't an
   option to customize the log level, `servoshell` has the `--log-filter` option on ohos targets, which allows
-  customizing the log filter of the `log` crate. 
-  By default servoshell sets a log filter which hides log statements from many crates, so you likely will need to 
+  customizing the log filter of the `log` crate.
+  By default servoshell sets a log filter which hides log statements from many crates, so you likely will need to
   set a custom log-filter if you aren't seeing the logs from the crate you are debugging.
 3. The `hilog` base log filter. `hdc shell hilog --base-level=<log_level>`. Can be combined with `--domain` and `--tag`
-  options to customize **which logs are saved**. 
+  options to customize **which logs are saved**.
 4. The `hilog` log filter when displaying logs: `hdc shell hilog --level=<level>`
 
 Most of the time option 2 and/or 4 should be used, since they allow quick changes with recompiling.
@@ -61,19 +62,27 @@ Most of the time option 2 and/or 4 should be used, since they allow quick change
 
 ### Hilog domains
 
-`hilog` allows setting a custom integer called "domain" (between 0 and 0xFFFF) when logging, which allows developers to easily filter logs using the domain. 
-For Rust code in servo logged via the `log` crate we set [`0xE0C3`] as the domain and for Spidermonkey C++ code we set [`0xE0C4`]. 
+`hilog` allows setting a custom integer called "domain" (between 0 and 0xFFFF) when logging, which allows developers to easily filter logs using the domain.
+For Rust code in servo logged via the `log` crate we set [`0xE0C3`] as the domain and for Spidermonkey C++ code we set [`0xE0C4`].
 These values are somewhat arbitrarily chosen.
 
 [`0xE0C3`]: https://github.com/servo/servo/blob/384d8f1ff895f070397b7a5a384428b1678416c5/ports/servoshell/egl/ohos.rs#L420
 [`0xE0C4`]: <TODO - link pending SM PR being merged.>
 
 ### Hilog privacy feature
 
-`hilog` has a privacy feature, which by default hides values in logs (e.g. from `%d` or `%s` substitutions). 
+`hilog` has a privacy feature, which by default hides values in logs (e.g. from `%d` or `%s` substitutions).
 Log statements from Rust are generally unaffected by this, since the string formatting is done on the Rust side.
 If you encounter this issue when viewing C/C++ logs, you can temporarily turn off the privacy feature by running:
 
 ```shell
 hdc shell hilog -p off
 ```
+
+### Devtools and port forwards
+
+You can enable the devtools and connect to them remotely with Firefox. It is easiest to do this with the command line via
+`hdc shell aa start -a EntryAbility -b org.servo.servo --psn=--devtools=1234`
+To connect to an instance of Servo you have to forward the port with
+`hdc fport tcp:1234 tcp:1234`. You should see a message that the forward succeeded. Now you can
+connect to the devtools using `localhost:1234`.