Balance sheet components - Summary of changed in allowance for sales incentives (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 30, 2017
Allowance for Sales Incentives [Roll Forward]
Beginning balance	$ 11,754	$ 11,195	$ 8,913
Charged to revenue	87,703	90,246	65,879
Utilization of sales incentive allowance	(79,406)	(89,687)	(63,597)
Ending balance	$ 20,051	$ 11,754	$ 11,195