linux-kernel - Re: [PATCH v4 09/10] arm64: dts: bcm2712: Add external clock for RP1 chipset on Rpi5

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Message-ID: <Z0Q7b-Qgjqs7NLS2@apocalypse>
Date: Mon, 25 Nov 2024 09:55:11 +0100
From: Andrea della Porta <andrea.porta@...e.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Andrea della Porta <andrea.porta@...e.com>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczynski <kw@...ux.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Bartosz Golaszewski <brgl@...ev.pl>,
	Derek Kiernan <derek.kiernan@....com>,
	Dragan Cvetic <dragan.cvetic@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Saravana Kannan <saravanak@...gle.com>, linux-clk@...r.kernel.org,
	devicetree@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-gpio@...r.kernel.org,
	Masahiro Yamada <masahiroy@...nel.org>,
	Stefan Wahren <wahrenst@....net>,
	Herve Codina <herve.codina@...tlin.com>,
	Luca Ceresoli <luca.ceresoli@...tlin.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v4 09/10] arm64: dts: bcm2712: Add external clock for RP1
 chipset on Rpi5

Hi Andrew,

On 20:41 Sun 24 Nov     , Andrew Lunn wrote:
> On Sun, Nov 24, 2024 at 11:51:46AM +0100, Andrea della Porta wrote:
> > The RP1 found on Raspberry Pi 5 board needs an external crystal at 50MHz.
> > Add clk_rp1_xosc node to provide that.
> 
> I'm wondering if this is the correct place for this clock. From your
> description, the bcm2712 itself does not provide the clock. There is a
> crystal on the board. So the board provides the clock. What happens
> when the RP1 is used on other boards? Also, does the RP1 need an
> actual crystal, or can you feed it a clock? Often such inputs are
> flexible, you can connect a crystal across two pins, or you can feed a
> clock into one pin.

AFAICT the only choice would be a crystal (I'll try to confirm that with
Rpi folksi, just to be sure).
In fact, I've expressed your same concern in the dicsussion that followed 
previous patchset revisions, and it seems that the preferred way is still
to stick to the current hw: since the crystal is on the same board as bcm2712,
it should not be described on the overaly.
I think you're right though on moving the clock definition to the board dts.
I'm still planning to define the clock in the rp1 overlay if any
new PCI card will contain it in the future: after all it will not cause any
clash with the clock defined in the board dts, which could be then removed
gradually.

Many thanks,
Andrea

> 
> If a crystal is the only choice, i would probably have it part of the
> RP1 overlay. If a clock can be used, i would make it a board property.
> 
> 	Andrew