linux-kernel - Re: [PATCH 01/25] serial: sh-sci: Update DT binding documentation for external clock input

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Date:	Thu, 19 Nov 2015 23:17:57 +0200
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Geert Uytterhoeven <geert+renesas@...der.be>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Simon Horman <horms@...ge.net.au>,
	Magnus Damm <magnus.damm@...il.com>,
	Yoshinori Sato <ysato@...rs.sourceforge.jp>,
	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 01/25] serial: sh-sci: Update DT binding documentation for external clock input

Hi Geert,

On Thursday 19 November 2015 21:39:50 Geert Uytterhoeven wrote:
> On Thu, Nov 19, 2015 at 9:27 PM, Laurent Pinchart wrote:
> > On Thursday 19 November 2015 22:19:14 Laurent Pinchart wrote:
> >> On Thursday 19 November 2015 19:38:40 Geert Uytterhoeven wrote:
> >> > Amend the DT bindings to include the optional external clock on
> >> > (H)SCI(F) and some SCIFA, where this pin can serve as a clock input,
> >> > depending on board wiring.
> >> > 
> >> > --- a/Documentation/devicetree/bindings/serial/renesas,sci-serial.txt
> >> > +++ b/Documentation/devicetree/bindings/serial/renesas,sci-serial.txt
> >> > 
> >> > @@ -43,6 +43,9 @@ Required properties:
> >> >    - clocks: Must contain a phandle and clock-specifier pair for each
> >> >      entry in clock-names.
> >> >    - clock-names: Must contain "fck" for the SCIx UART functional
> >> >    clock.
> >> > +    On (H)SCI(F) and some SCIFA, an additional clock may be specified:
> >>
> >> Could you list the SCIFA variants that support external clocks ?
> >> 
> >> > +      - "hsck" for the optional external clock input (on HSCIF),
> >> > +      - "sck" for the optional external clock input (on other
> >> > variants).
> > 
> > Additionally, those clocks are used as inputs to the baud rate generator
> > for external clocks, as the ones listed in patch 02/25 in this series.
> > I'd merge the two patches and clarify the wording.
> 
> "SCK" predates the BRG, it even exists on SCI in H8/300.
> 
> That SCK is used as input to the BRG is just an artefact of how the BRG was
> added to the SCIF. The BRG is just muxed with the existing SCK to form a
> clock input, which is muxed with the BRR clock through the SCSCR.CKEx bits.
> And the BRG itself can choose between SCIF_CLK and INT_CLK.
> 
> Hence that's why I split it in two parts.

It makes sense with the explanation.

I think some of the patches should be clarified to mention BRG-EC (or whatever 
you want to call it) instead of just BRG, as otherwise it's very easy to 
confuse the two BRGs. The (H)SCK clock is an input to the internal BRG, while 
the SCIF_CLK and INT_CLK are inputs to the BRG-EC. Without clarification the 
DT bindings and the code can be hard to understand.

-- 
Regards,

Laurent Pinchart

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