Balance sheet components - Change in deferred revenue (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Contract with Customer, Liability [Roll Forward]
Deferred revenue, beginning of period	$ 50,967	$ 45,567
Recognition of revenue included in beginning of period deferred revenue	(6,040)	(6,014)
Revenue deferred, net of revenue recognized on contracts in the respective period	10,306	9,831
Deferred revenue, end of period	$ 55,233	$ 49,384