Forcing display size on Pi4

Forcing display size on Pi4

[Lyndon Nerenberg]

I have a 2K display that Plan9 forces to 1920x1080 resolution.
Poking around 9/bcm/screen.c indicates that setting vgasize=WxHxD
should force the size, but adding a suitable entry in cmdline.txt
just gives me a blank display.

Before I dig deeper, is this expected to work?  If it is I'll start
adding some kernel prints and try to find out what's not working.

--lyndon

Re: [9fans] Forcing display size on Pi4

[Richard Miller]

> I have a 2K display that Plan9 forces to 1920x1080 resolution.
> Poking around 9/bcm/screen.c indicates that setting vgasize=WxHxD
> should force the size, but adding a suitable entry in cmdline.txt
> just gives me a blank display.
> 
> Before I dig deeper, is this expected to work?

Possibly not.  I haven't tried vgasize= for years and there have been a
lot of changes to the firmware & videocore mailbox interface since then.

Plan 9 should accept whatever resolution comes from the gpu, so it
may be the firmware which "forces" 1920x1080.  Maybe gpu_mem needs
to be bigger than the usual 16, or some other config.txt adjustment
is needed?

Re: [9fans] Forcing display size on Pi4

[Bakul Shah]

> On Dec 17, 2019, at 1:31 PM, Richard Miller <9fans@hamnavoe.com> wrote:
> 
>> I have a 2K display that Plan9 forces to 1920x1080 resolution.
>> Poking around 9/bcm/screen.c indicates that setting vgasize=WxHxD
>> should force the size, but adding a suitable entry in cmdline.txt
>> just gives me a blank display.
>> 
>> Before I dig deeper, is this expected to work?
> 
> Possibly not.  I haven't tried vgasize= for years and there have been a
> lot of changes to the firmware & videocore mailbox interface since then.
> 
> Plan 9 should accept whatever resolution comes from the gpu, so it
> may be the firmware which "forces" 1920x1080.  Maybe gpu_mem needs
> to be bigger than the usual 16, or some other config.txt adjustment
> is needed?

I was able run earlier pis with a 4K@30Hz display but haven't tried this
with pi4. I had to use some custom parameters.

https://www.raspberrypi.org/documentation/configuration/hdmi-config.md

says to use hdmi_enable_4kp60=1 in config.txt if you have a 4k@60hz display.
You can choose custom modes as well. There is a config_hdmi_boost parameter
to increase HDMI drive strength but this doesn't work for pi4.

Re: [9fans] Forcing display size on Pi4

[hiro]

here 4k@60Hz works fine with rpi4.
nothing plan9 specific was needed.
i copied the fat32 content from raspbian and adjusted the config.txt
according to the docs to make it work. 9front release had some
firmware (or dtd files, or something) that was either too old or
missing