Thank you Jake Lo,
I have tried your SSDT, no luck, the screen still flickering.
But I have found this topic https://www.insanelymac.com/forum/topic/299911-dell-inspiron-3x21-5x21-clover-install-guide-dell and tried to patch:
1. DSDT by the file https://www.dropbox.com/s/8udi7ki1qxgl1iz/Dell Exx21 1600x900 patches.txt
2. SSDT-4-SgTabl by
into definitionblock code_regex External\s+\((.*_SB_\.PCI0\.GFX0\.DD02\._BCM),\s+IntObj\) replace_matched begin External(%1, MethodObj) end;
into method label _BCM parent_label DD02 code_regex Return\s+\(([^\)]*)\)\n.*Arg0 replace_matched begin %1(Arg0) end;
# rename GFX0
into_all all code_regex \.GFX0 replaceall_matched begin .IGPU end;
into_all all label \_SB.PCI0.GFX0 set_label begin \_SB.PCI0.IGPU end;
# call _OFF from _SB.PCI0.PEG0.PEGP
into method label _INI parent_label \_SB.PCI0.PEG0.PEGP code_regex . insert begin _OFF()\n end;
into definitionblock code_regex . insert
begin
External(\_SB_.PCI0.PEG0.PEGP._OFF, MethodObj)\n
end;
3. SSDT-5-OptTabl by
into_all all code_regex pnp0c14 replaceall_matched begin PNP0C14 end;
into definitionblock code_regex External\s+\((.*_SB_\.PCI0\.GFX0\.DD02\._BCM),\s+IntObj\) replace_matched begin External(%1, MethodObj) end;
into method label _BCM parent_label DD02 code_regex Return\s+\(([^\)]*)\)\n.*Arg0 replace_matched begin %1(Arg0) end;
into method label WMMX code_regex If\s\(CondRefOf\s\(\\_SB\.PCI0\.GFX0\._DSM[^}]*\} replace_matched begin //nothing end;
# rename GFX0
into_all all code_regex \.GFX0 replaceall_matched begin .IGPU end;
into_all all label \_SB.PCI0.GFX0 set_label begin \_SB.PCI0.IGPU end;
4. Run the command in the terminal
sudo perl -pi -e 's|\x05\x03\x00\x00\x02\x00\x00\x00\x30\x02\x00\x00|\x05\x03\x00\x00\x02\x00\x00\x00\x30\x00\x00\x00|' /System/Library/Extensions/AppleIntelFramebufferCapri.kext/Contents/MacOS/AppleIntelFramebufferCapri
The problem has been solved, no flickering screen. Any one who want to install Sierra on Dell N3421 may use the attached EFI folder for reffering
Credit to:
1. sugarface (from insanelymac.com) for patching guide
2. Jake Lo and Hervé for clover configuration
EFI.zip