linux-kernel - Re: [PATCH 19/19] Documentation: ACPI for ARM64

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Message-ID: <5959830.lpqeqL7HbK@wuerfel>
Date:	Mon, 28 Jul 2014 12:46:17 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Graeme Gregory <graeme.gregory@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Olof Johansson <olof@...om.net>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Mark Rutland <mark.rutland@....com>,
	Mark Brown <broonie@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Lv Zheng <lv.zheng@...el.com>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Robert Moore <robert.moore@...el.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	Grant Likely <grant.likely@...aro.org>,
	Charles Garcia-Tobin <Charles.Garcia-Tobin@....com>,
	Robert Richter <rric@...nel.org>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>,
	Liviu Dudau <Liviu.Dudau@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sudeep Holla <Sudeep.Holla@....com>
Subject: Re: [PATCH 19/19] Documentation: ACPI for ARM64

On Monday 28 July 2014 10:23:57 Graeme Gregory wrote:
> The PL011 UART is the use-case I keep hitting, that IP block has a 
> variable input clock on pretty much everything I have seen in the wild. 

Ok, I see. What does ACPI-5.1 say about pl011?

Interestingly, the subset of pl011 that is specified by SBSA does not
contain the IBRD/FBRD registers, effectively making it a fixed-rated
UART (I guess that would be a ART, without the U then), and you
consequently don't even need to know the clock rate.

However, my guess is that most hardware in the real world contains
an actual pl011 and it does make a lot of sense to allow setting
the baud rate on it, which then requires knowing the input clock.

If there is any hardware that implements just the SBSA-mandated subset
rather than the full pl011, we should probably implement both
in the kernel: a dumb driver that can only send and receive, and the
more complex one that can set the bit rates and flow-control but that
requires a standardized ACPI table with the input clock rate.

Whether the two would belong into one file or two separate driver
modules is something I can't tell, it would be up to the serial
maintainers to decide.

> I really hope that this use does not spread beyond a few essential 
> devices like the UART. IMO all real hardware should be the other side of 
> a PCIe bridge.

I would definitely agree with that.

	Arnd
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