How to follow curr file's project root dir?

[qiine]

The idea is to find the project's root dir based on current file, when entering a buffer and set neotree's root accordingly IF neotree is already open.
But it seems more complicated than I thought.

I have tried this:

     vim.api.nvim_create_autocmd('BufEnter', {
                callback = function()
                local fp =  vim.api.nvim_buf_get_name(0)
                local rootdir = vim.fs.root(fp, {'README.md', "Makefile", ".git"})

                require("neo-tree.command").execute({
                    reveal = false,
                    focus = false,
                    dir = rootdir
                })
            end,
        })

But it does not work properly because it constantly open and try re-focus neotree, which puzzle me..

[qiine]

In fact I realize that I do not know how to set neotree's root dir without opening it in lua.