feat(fbdev): Allow forcing refresh

[Johennes]

Description of the feature or fix

A clear and concise description of what the bug or new feature is. This feature exists in the old fbdev driver in lv_drivers.

Notes

• Update the Documentation if needed.
• Add Examples if relevant.
• Add Tests if applicable.
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[kisvegabor]

What the problem does it resolves? Tearing?

[Johennes]

Sorry, I was mistaken. This didn't exist in lv_drivers upstream. We had only made the change on our fork. The issue that spawned this is https://gitlab.com/cherrypicker/unl0kr/-/issues/34. On some devices the screen would stay black or only render the area that had changed. The problem doesn't occur anymore when using direct rendering together with the force refresh feature from this PR.

[kisvegabor]

I see, thank you! Would it make sense to enable it by default?

[Johennes]

No, I wouldn't recommend that actually. When I enable this on my laptop, it's causing a noticeable performance impact. It's not negatively noticeable on the device affected by the issue. But I'd definitely only turn it on when needed. In our app we've just made it a config setting.

[kisvegabor]

All clear, thank you. Final question: would you be interested in adding docs for the frame buffer device driver? Similar to this.

[Johennes]

Good idea. I've added some rudimentary docs.

[kisvegabor]

It was quick! Thank you! 🙂