How to format elixir files with elixirls by default?

I use elixirls lsp-client and efm-server (mainly for credo) for working with elixir-files in Neovim. I want to use lsp-config for formatting elixir files and want to make it default lsp-client for vim.lsp.buf.formatting() command.

Currently, everytime I try to save a file, it asks me to choose a language server as shown in the image below.

How to make elixirls the default language server for vim.lsp.buf.formatting() and vim.lsp.buf.formatting_sync(nil, 100) commands ?

My relevant init.lua file is as follows:

-----------------------------------------------------------------------------------------------
---  LSP
-----------------------------------------------------------------------------------------------
local nvim_lsp = require("lspconfig")

-- Use an on_attach function to only map the following keys
-- after the language server attaches to the current buffer
local custom_attach = function(client, bufnr)
    local function buf_set_keymap(...)
        vim.api.nvim_buf_set_keymap(bufnr, ...)
    end
    local function buf_set_option(...)
        vim.api.nvim_buf_set_option(bufnr, ...)
    end

    -- Enable completion triggered by <c-x><c-o>
    buf_set_option("omnifunc", "v:lua.vim.lsp.omnifunc")

    -- Mappings.
    local opts = {noremap = true, silent = true}

    -- See `:help vim.lsp.*` for documentation on any of the below functions
    buf_set_keymap("n", "gD", "<Cmd>lua vim.lsp.buf.declaration()<CR>", opts)
    buf_set_keymap("n", "gd", "<Cmd>lua vim.lsp.buf.definition()<CR>", opts)
    buf_set_keymap("n", "K", "<Cmd>lua vim.lsp.buf.hover()<CR>", opts)
    buf_set_keymap("n", "gi", "<cmd>lua vim.lsp.buf.implementation()<CR>", opts)
    buf_set_keymap("n", "<C-k>", "<cmd>lua vim.lsp.buf.signature_help()<CR>", opts)
    buf_set_keymap("n", "<space>wa", "<cmd>lua vim.lsp.buf.add_workspace_folder()<CR>", opts)
    buf_set_keymap("n", "<space>wr", "<cmd>lua vim.lsp.buf.remove_workspace_folder()<CR>", opts)
    buf_set_keymap("n", "<space>wl", "<cmd>lua print(vim.inspect(vim.lsp.buf.list_workspace_folders()))<CR>", opts)
    buf_set_keymap("n", "<space>D", "<cmd>lua vim.lsp.buf.type_definition()<CR>", opts)
    buf_set_keymap("n", "<space>rn", "<cmd>lua vim.lsp.buf.rename()<CR>", opts)
    buf_set_keymap("n", "<space>ca", "<cmd>lua vim.lsp.buf.code_action()<CR>", opts)
    buf_set_keymap("n", "gr", "<cmd>lua vim.lsp.buf.references()<CR>", opts)
    buf_set_keymap("n", "<space>e", "<cmd>lua vim.lsp.diagnostic.show_line_diagnostics()<CR>", opts)
    buf_set_keymap("n", "[d", "<cmd>lua vim.lsp.diagnostic.goto_prev()<CR>", opts)
    buf_set_keymap("n", "]d", "<cmd>lua vim.lsp.diagnostic.goto_next()<CR>", opts)
    buf_set_keymap("n", "<space>q", "<cmd>lua vim.lsp.diagnostic.set_loclist()<CR>", opts)
    buf_set_keymap("n", "<space>f", "<cmd>lua vim.lsp.buf.formatting_sync(nil, 100)<CR>", opts)

    -- vim.api.nvim_command("au BufWritePost <buffer> lua vim.lsp.buf.formatting()")
    -- vim.api.nvim_command("au BufWritePost <buffer> lua vim.lsp.buf.formatting_sync(nil, 100)")
    -- autocmd BufWritePre *.lua lua vim.lsp.buf.formatting_sync(nil, 100)
end

USER = vim.fn.expand("$USER")
local sumneko_binary = "/Users/" .. USER .. "/language-servers/lua-language-server/bin/macOS/lua-language-server"
local sumneko_root_path = "/Users/" .. USER .. "/language-servers/lua-language-server"
nvim_lsp.sumneko_lua.setup {
    cmd = {sumneko_binary, "-E", sumneko_root_path .. "/main.lua"},
    on_attach = custom_attach,
    settings = {
        Lua = {
            runtime = {
                -- Tell the language server which version of Lua you're using (most likely LuaJIT in the case of Neovim)
                version = "LuaJIT",
                -- Setup your lua path
                path = vim.split(package.path, ";")
            },
            diagnostics = {
                -- Get the language server to recognize the `vim` global
                globals = {"vim"}
            },
            workspace = {
                -- Make the server aware of Neovim runtime files
                library = {[vim.fn.expand("$VIMRUNTIME/lua")] = true, [vim.fn.expand("$VIMRUNTIME/lua/vim/lsp")] = true}
            }
        }
    }
}

vim.api.nvim_command("au BufWritePost *.lua lua vim.lsp.buf.formatting()")
vim.api.nvim_command("au BufWritePost *.ex lua vim.lsp.buf.formatting()")
-- vim.api.nvim_command("au BufWritePost *.exs lua vim.lsp.buf.formatting()")

require"lspconfig".efm.setup {
    init_options = {documentFormatting = true},
    filetypes = {"lua", "elixir"},
    settings = {
        rootMarkers = {".git/"},
        languages = {
            lua = {
                {
                    formatCommand = "lua-format -i --no-keep-simple-function-one-line --no-break-after-operator --column-limit=150 --break-after-table-lb",
                    formatStdin = true
                }
            }
        }
    }
}

local path_to_elixirls = vim.fn.expand("/Users/" .. USER .. "/language-servers/elixir-ls/rel/language_server.sh")
nvim_lsp.elixirls.setup({
    cmd = {path_to_elixirls},
    on_attach = custom_attach,
    settings = {
        elixirLS = {
            -- I choose to disable dialyzer for personal reasons, but
            -- I would suggest you also disable it unless you are well
            -- aquainted with dialzyer and know how to use it.
            dialyzerEnabled = false,
            -- I also choose to turn off the auto dep fetching feature.
            -- It often get's into a weird state that requires deleting
            -- the .elixir_ls directory and restarting your editor.
            fetchDeps = false
        }
    }
})

Solution

Using vim.lsp.buf.formatting_seq_sync() instead of vim.lsp.buf.formatting() fixes the issue.