Balance sheet components - Change in deferred revenue (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 28, 2020	Mar. 30, 2019
Contract with Customer, Liability [Roll Forward]
Deferred revenue, beginning of period	$ 56,449	$ 50,967
Recognition of revenue included in beginning of period deferred revenue	(6,904)	(6,040)
Revenue deferred, net of revenue recognized on contracts in the respective period	10,798	10,306
Deferred revenue, end of period	$ 60,343	$ 55,233