linux-kernel - Re: [PATCH 1/2] ARM: dts: bcm5301x: Add BCM SVK DT files

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Date:	Wed, 14 Oct 2015 11:36:22 -0400
From:	Jon Mason <jonmason@...adcom.com>
To:	Ray Jui <rjui@...adcom.com>
CC:	Hauke Mehrtens <hauke@...ke-m.de>,
	Florian Fainelli <f.fainelli@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"Russell King" <linux@....linux.org.uk>,
	<devicetree@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>,
	<bcm-kernel-feedback-list@...adcom.com>,
	Rafał Miłecki <zajec5@...il.com>
Subject: Re: [PATCH 1/2] ARM: dts: bcm5301x: Add BCM SVK DT files

On Tue, Oct 13, 2015 at 03:43:20PM -0700, Ray Jui wrote:
> 
> 
> On 10/13/2015 2:38 PM, Jon Mason wrote:
> > On Sat, Oct 10, 2015 at 04:39:00PM +0200, Hauke Mehrtens wrote:
> >> On 10/03/2015 12:22 AM, Jon Mason wrote:
> >>> Add device tree files for Broadcom Northstar based SVKs.  Since the
> >>> bcm5301x.dtsi already exists, all that is necessary is the dts files to
> >>> enable the UARTs (and specify the RAM size for the 4708/9).  With these
> >>> files, the SVKs are able to boot to shell.
> >>>
> >>> Signed-off-by: Jon Mason <jonmason@...adcom.com>
> >>> ---
> >>>  arch/arm/boot/dts/Makefile       |  5 +++-
> >>>  arch/arm/boot/dts/bcm94708.dts   | 52 +++++++++++++++++++++++++++++++++++
> >>>  arch/arm/boot/dts/bcm94709.dts   | 52 +++++++++++++++++++++++++++++++++++
> >>>  arch/arm/boot/dts/bcm953012k.dts | 59 ++++++++++++++++++++++++++++++++++++++++
> >>>  4 files changed, 167 insertions(+), 1 deletion(-)
> >>>  create mode 100644 arch/arm/boot/dts/bcm94708.dts
> >>>  create mode 100644 arch/arm/boot/dts/bcm94709.dts
> >>>  create mode 100644 arch/arm/boot/dts/bcm953012k.dts
> >>>
> 
> >>> +&uart0 {
> >>> +	clock-frequency = <62499840>;
> >>
> >> Just out of curiosity on what does this clock frequency depend? I saw
> >> your patch adding a real clock driver which should make this obsolete.
> >  
> > Better to add this now, as the device tree part might be out of sync
> > for a time.
> 
> Sure, this can potentially be a reason to not using the real clock node
> and just use a hardcoded clock frequency for now, until the other clock
> change is merged, then you can submit another patch to use it.
> 
> Also, is it not better to make the UARTs a static clock
> > and not dependent on the clk driver?
> > 
> 
> I disagree. You should always use the real clock driver for querying the
> clock frequency, in the case when the clock driver is available.
> 
> Today one can change the core clock for UART in the bootloader for
> various reasons (and we saw that happening a lot in the past), which in
> this case will make your kernel not seeing any console output.
> 
> By always querying the clock rate based on real registers instead of a
> hardcoded value, you don't have that issue and your kernel is less
> dependent on any changes in the bootloader.

Fair enough.  This is moot until the clk driver changes get pulled in.
This file can be changed at that time :)

Thanks,
Jon


> 
> > Thanks,
> > Jon
> > 
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