Balance sheet components - Finite-lived intangible assets, future amortization expense (Details) - USD ($) $ in Thousands	6 Months Ended
	Mar. 28, 2020	Sep. 28, 2019
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	$ 7,566
Accumulated amortization	(536)
Finite-lived intangible assets, net carrying value	7,030
Intangible assets, gross carrying amount (excluding goodwill)	27,679
Intangible assets, net carrying value (excluding goodwill)	$ 27,143	$ 13
Weighted-average remaining life	4 years 4 months 2 days
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Remainder of fiscal 2020	$ 924
2021	1,848
2022	1,832
2023	1,235
2024	1,020
2025	171
Finite-lived intangible assets, net carrying value	7,030
In progress research and development
Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets (excluding goodwill)	20,113
Technology
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	7,527
Accumulated amortization	(529)
Finite-lived intangible assets, net carrying value	$ 6,998
Weighted-average remaining life	4 years 3 months 29 days
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Finite-lived intangible assets, net carrying value	$ 6,998
Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross carrying amount	39
Accumulated amortization	(7)
Finite-lived intangible assets, net carrying value	$ 32
Weighted-average remaining life	3 days
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Finite-lived intangible assets, net carrying value	$ 32