[9fans] Seiki 4k + RPi + Plan9

[Bakul Shah <bakul@bitblocks.com>]

Given that the Seiki 39" 4k (3840x2160) display prices have
come down quite a bit (now $329 @ amazon) I wanted to see if
I can use it with the RPi & plan9 as well.

1. You will need one plan9 fix:

% diffy /sys/src/9/bcm/mmu.c
307c307
<	cachedwbse(pte0, pte - pte0);
---
>	cachedwbse(pte0, (pte - pte0)*sizeof(PTE));

2. You will need to upgrade firmware to at least June 18
version (/sources seems down at the moment but the image
Richard put up at /n/sources/contrib/miller/9pi.img.gz has
it). Or follow instructions from the RPi website.

3. You will need to fiddle with config.txt.

gpu_mem=64
framebuffer_width=3840
framebuffer_height=2160
hdmi_group=2
hdmi_mode=87
hdmi_cvt 3840 2160 15
max_framebuffer_width=3840
max_framebuffer_height=2160
hdmi_pixel_freq_limit=400000000
disable_overscan=1

I also needed

config_hdmi_boot=4

to boost the hdmi signal. You may not.

That is it!

Some caveats:
Its screen feels huge! I sit about 30" from it. It may be too
big for a small or shallow office. It may be hard on your eyes
or neck or back. I still haven't found the right height for
it and may end up installing it on a wall.

I have used it for a couple of weeks with a MBP but hardly
at all as a plan9 terminal.

I can run it at 30Hz at 4k resolution with a recent MBP but
this resolution does not work with a late 2011 MBP.

At 30hz some people find its cursor to be laggy. Didn't
bother me.

As per a RPi forum thread this pixel clock should work upto
24Hz. For 24Hz change the hdmi_cvt line to
    hdmi_cvt 3840 2160 24
The setting I used is for 15Hz and the cursor is a bit laggy.

You will probably need to install new firmware on the Seiki
and change some of its default settings.