Indeed that feature would be optimal. Just to confirm, you have replaced force_thresh_dma_mode with this txpbl thing, right? That is the only ethernet modification I have in the dts, done though u-boot. That is on linux 5.1
fdt rm /ethernet@ff540000 snps,force_thresh_dma_mode fdt set /ethernet@ff540000 snps,txpbl <0x4>