Balance sheet components - Change in deferred revenue (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 29, 2019	Jun. 30, 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	(8,827)	(8,367)
Revenue deferred, net of revenue recognized on contracts in the respective period	13,966	18,594
Deferred revenue, end of period	$ 56,106	$ 55,794