synaptiko/01-how-to-modify-physical-display-size-in-edid.md

## 01-how-to-modify-physical-display-size-in-edid.md

      
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              01-how-to-modify-physical-display-size-in-edid.md
            
          
    How to modify physical display size in EDID

Following method describes how to fix EDID display size information of your monitor if it's wrong (some of Samsung's monitors report incorrect size when connected over HDMI, for example).
It assumes that your monitor is connected to HDMI1 port (HDMI-A-1 is full kernel address).
I used following tools:

Bless
edid-decode-git
edid-rw (I used parts of this script for edid-checksum.py)
read-edid (parse-edid, get-edid)


get the current edid file

cat /sys/class/drm/card0-HDMI-A-1/edid > incorrect-edid.bin


find display size saved in your edid file

parse-edid < incorrect-edid.bin | grep DisplaySize

you should see something like:
Checksum Correct
        DisplaySize 160 90

it means your monitor tells you it has 160mm×90mm size

update size information


display size lives on offset 0x15 (width) and 0x16 (height) of the binary file
it's in centimeters! it means 160mm -> 16cm -> 0x10 and 90mm -> 9cm -> 0x09
open the binary file in hexadecimal editor (I used Bless)
find the index and verify that it's the incorrect size you want to fix (in my case 0x10 on 0x15 offset and 0x09 on 0x16 offset)
delete these two numbers and type new ones instead, 530mm -> 53cm -> 0x35 and 300mm -> 30cm -> 0x1E in my case


save as correct-edid.py and update checksum

create edid-checksum.py (see below)
chmod u+x edid-checksum.py
./edid-checksum.py < correct-edid.bin

you should see that checksum is now invalid:
You need to fix checksum on offset 0x7f
0x75 is BAD, should be 0x65

update the checksum on offset 0x7f to value 0x65; save and check again, you should see:
Checksums are correct!


copy fixed edid file

sudo cp correct-edid.bin /usr/lib/firmware/edid/syncmaster-2494hm.bin


use it in your kernel; there are two parts:


update your /etc/mkinitcpio.conf (if you want to have it early when booting)
add your graphics driver to modules


MODULES=( … i915 … )

add binary file to files which are added to your initramfs
FILES=(/usr/lib/firmware/edid/syncmaster-2494hm.bin)

regenerate your initramfs
sudo mkinitcpio -p linux


add kernel options to your bootloader, I use systemd bootloader so I updated /boot/loader/entries/arch.conf

options drm_kms_helper.edid_firmware=HDMI-A-1:edid/syncmaster-2494hm.bin …


reboot and be happy again!!!

Do it on your own risk!!! You can really mess up your system if you don't know what exactly you are doing!

  
## edid-checksum.py
#!/usr/bin/env python
import os, sys, array

EDID_LEN = (128, 256)

def getlen(edidhdr):
    if edidhdr[18] <= 0 or edidhdr[18] > len(EDID_LEN):
        sys.exit('ERROR: Unknown EDID version %d' % edidhdr[18])

    return EDID_LEN[edidhdr[18] - 1]

def checksum(edid):
    return (0 - (sum(edid[:-1]) % 256)) % 256

def main():
    edid = array.array('B', os.read(sys.stdin.fileno(), 256)).tolist()
    edid_len = getlen(edid)
    edid_n = edid[edid_len - 2]
    nbytes = edid_len + (edid_len * edid_n)

    if len(edid) != nbytes:
        sys.exit('ERROR: Input must be %d bytes' % nbytes)

    for b in range(edid_n + 1):
        x = (b + 1) * edid_len
        actsum = edid[x - 1]
        calsum = checksum(edid[x - edid_len:x])

        if actsum != calsum:
            sys.exit('You need to fix checksum on offset 0x%02x\n'
                    '0x%02x is BAD, should be 0x%02x' % (x - 1, actsum, calsum))

    sys.exit('Checksums are correct!')

if __name__ == '__main__':
    main()
	#!/usr/bin/env python
	import os, sys, array

	EDID_LEN = (128, 256)

	def getlen(edidhdr):
	if edidhdr[18] <= 0 or edidhdr[18] > len(EDID_LEN):
	sys.exit('ERROR: Unknown EDID version %d' % edidhdr[18])

	return EDID_LEN[edidhdr[18] - 1]

	def checksum(edid):
	return (0 - (sum(edid[:-1]) % 256)) % 256

	def main():
	edid = array.array('B', os.read(sys.stdin.fileno(), 256)).tolist()
	edid_len = getlen(edid)
	edid_n = edid[edid_len - 2]
	nbytes = edid_len + (edid_len * edid_n)

	if len(edid) != nbytes:
	sys.exit('ERROR: Input must be %d bytes' % nbytes)

	for b in range(edid_n + 1):
	x = (b + 1) * edid_len
	actsum = edid[x - 1]
	calsum = checksum(edid[x - edid_len:x])

	if actsum != calsum:
	sys.exit('You need to fix checksum on offset 0x%02x\n'
	'0x%02x is BAD, should be 0x%02x' % (x - 1, actsum, calsum))

	sys.exit('Checksums are correct!')

	if __name__ == '__main__':
	main()