Hi,
Combuster wrote:I started working on it
Here - Still needs a lot of work, but there's a nice introduction for everyone that wonders what this is all about.
Looks like a very promising beginning!
Just to let you know, I'm not happy with those formulas yet, and plan to continue working on them.
I think part of the problem is that the original GTF formulas from VESA are copied from their spreadsheet, so that (for a silly example) instead of doing something like:
Code: Select all
frequency = 10000000
period = 1 / frequency seconds
They'll do:
Code: Select all
frequency_in_MHz = 10
period_in_ms = 1 / (frequency_in_MHz * 1000000) * 1000
And then simplify it to:
Code: Select all
frequency_in_MHz = 10
period_in_ms = 1 / frequency_in_MHz / 1000
And then they'll remove any indication of what they've done:
Code: Select all
frequency = 10
period = 1 / frequency / 1000
So that in the end, the formulas from VESA end up being a confusing mess where you're never too sure if there's a hidden scaling factor or not. Another simple example (that isn't made up) is "MARGINS_PERCENT", which is a value that would range from 0 to 50 that's typically divided by 100; and isn't a value that ranges from 0 to 0.5 that doesn't need scaling.
I plan on going through the formulas again, and making sure that all variables are either in pixels, lines, seconds or hertz (with no implied/hidden milliseconds, microseconds, kilohertz, megahertz or "percent" scaling factors).
I've also noticed that some of the calculations are the same in each formula. Rather than having 3 independent/larger pieces of code, it can be split into 3 smaller pieces of code that all use the same subroutines for common things. For example, you'd be able to replace about 10 lines from each formula with "call do_precalculations" to avoid code duplication (triplication?).
Also, I only posted "primary formulas". There's also a "secondary formula" where the results from the first set of calculations are used to find additional parameters; and there's another calculation (called the "blanking formula") for finding the values for "C_PRIME" and "M_PRIME" from some variables called C, M, J and K that's used for "secondary timings". I want to do the same for these formulas as I'm doing for the primary calculations. Then I need to go through all of these parameters and figure out which ones are actually useful (both for setting a video mode using VBE, and for setting a video mode in a "bare metal" video driver), and then work backwards removing redundant calculations (i.e. calculations that are only used to create information that you don't need anyway). After I've done this, I can combine it with the "common subroutines" idea, so that the entire "secondary formula" ends up being part of the "do_postcalculations()" routine, and so that the entire blanking formula becomes part of the "do_precalculations()" routine.
Then I want to find the valid ranges for all variables used in all calculations. This is mostly because of how I'm planning to implement it. For example, if I knew that "H_FREQ" ranges from 50000 to 1000000 then I'd be able to use "20:12" fixed point for this variable to improve the accuracy I get from a 32-bit integer.
Finally, all of these calculations can be simplified a lot if you assume that the "default GTF values" are being used. I want to provide simplified versions of the formulas, because most of the time you can use the default GTF values and can skip a lot of work.
Cheers,
Brendan
