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Date: Wed, 15 Jan 2020 20:54:15 +0100 From: Thomas Gleixner <tglx@...utronix.de> To: Paul Cercueil <paul@...pouillou.net>, Maarten ter Huurne <maarten@...ewalker.org> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>, od@...c.me, linux-kernel@...r.kernel.org, Mathieu Malaterre <malat@...ian.org>, Artur Rojek <contact@...ur-rojek.eu> Subject: Re: [PATCH v3] clocksource: Add driver for the Ingenic JZ47xx OST Paul Cercueil <paul@...pouillou.net> writes: > Le mer., janv. 15, 2020 at 18:48, Maarten ter Huurne > <maarten@...ewalker.org> a écrit : >> On Wednesday, 15 January 2020 14:57:01 CET Paul Cercueil wrote: >>> Le mer., janv. 15, 2020 at 14:44, Daniel Lezcano >>> <daniel.lezcano@...aro.org> a écrit : >>> > Is the JZ47xx OST really a mfd needing a regmap? (Note regmap_read >>> > will take a lock). >>> >>> Yes, the TCU_REG_OST_TCSR register is shared with the clocks driver. >> >> The TCU_REG_OST_TCSR register is only used in the probe though. >> >> To get the counter value from TCU_REG_OST_CNTL/TCU_REG_OST_CNTH you >> could technically do it by reading the register directly, if >> performance >> concerns make it necessary to bypass the usual kernel infrastructure >> for >> dealing with shared registers. > > In theory yes, in practice there's no easy way to do that (the > underlying mmio pointer is not obtainable from the regmap), and > besides, the lock is just a spinlock and not a mutex. That lock still a massive contention point as clock readouts can be pretty frequent depending on workloads. Just think about tracing ... So I really would avoid both the lock and that ugly 64bit readout thing. Thanks, tglx
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