linux-kernel - Re: [PATCH v8 03/10] clk: eyeq5: add platform driver, and init routine at of_clk

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Message-Id: <CZHOPWYS6IBQ.RFB7JANYC769@bootlin.com>
Date: Thu, 29 Feb 2024 16:57:28 +0100
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Andy Shevchenko" <andriy.shevchenko@...el.com>
Cc: "Gregory CLEMENT" <gregory.clement@...tlin.com>, "Michael Turquette"
 <mturquette@...libre.com>, "Stephen Boyd" <sboyd@...nel.org>, "Rob Herring"
 <robh+dt@...nel.org>, "Krzysztof Kozlowski"
 <krzysztof.kozlowski+dt@...aro.org>, "Conor Dooley" <conor+dt@...nel.org>,
 "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>, "Linus Walleij"
 <linus.walleij@...aro.org>, Rafał Miłecki
 <rafal@...ecki.pl>, "Philipp Zabel" <p.zabel@...gutronix.de>, "Vladimir
 Kondratiev" <vladimir.kondratiev@...ileye.com>,
 <linux-mips@...r.kernel.org>, <linux-clk@...r.kernel.org>,
 <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>, "Thomas
 Petazzoni" <thomas.petazzoni@...tlin.com>, "Tawfik Bayouk"
 <tawfik.bayouk@...ileye.com>, <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH v8 03/10] clk: eyeq5: add platform driver, and init
 routine at of_clk_init()

Hello,

On Thu Feb 29, 2024 at 4:48 PM CET, Andy Shevchenko wrote:
> On Thu, Feb 29, 2024 at 04:40:25PM +0100, Théo Lebrun wrote:
> > On Thu Feb 29, 2024 at 3:59 PM CET, Andy Shevchenko wrote:
> > > On Thu, Feb 29, 2024 at 03:27:01PM +0100, Théo Lebrun wrote:
> > > > On Wed, Feb 28, 2024 at 03:33:29PM +0100, Théo Lebrun wrote:
> > > > > On Tue Feb 27, 2024 at 6:11 PM CET, Andy Shevchenko wrote:
> > > > > > On Tue, Feb 27, 2024 at 03:55:24PM +0100, Théo Lebrun wrote:
>
> [...]
>
> > > > > > > > +	u32		reg;	/* next 8 bytes are r0 and r1 */
> > > > > > >
> > > > > > > Not sure this comments gives any clarification to a mere reader of the code.
> > > > > > > Perhaps you want to name this as reg64 (at least it will show that you have
> > > > > > > 8 bytes, but I have no clue what is the semantic relationship between r0 and
> > > > > > > r1, it's quite cryptic to me). Or maybe it should be reg_0_1?
> > > > > > 
> > > > > > Clocks are defined by two 32-bit registers. We only store the first
> > > > > > register offset because they always follow each other.
> > > > >
> > > > > > I like the reg64 name and will remove the comment. This straight forward
> > > > > > code is found in the rest of the code, I don't think it is anything
> > > > > > hard to understand (ie does not need a comment):
> > > > > > 
> > > > > > 	u32 r0 = readl(base_plls + pll->reg);
> > > > > > 	u32 r1 = readl(base_plls + pll->reg + sizeof(r0));
> > > > >
> > > > > Btw, why readq()/writeq() (with probably the inclusion of io-64-nonatomic-lo-hi.h)
> > > > > can be used in this case? It will be much better overall and be aligned with
> > > > > reg64 name.
> > > > 
> > > > The doc talks in terms of 32-bit registers. I do not see a reason to
> > > > work in 64-bit. If we get a 64-bit value that we need to split we need
> > > > to think about the endianness of our platform, which makes things more
> > > > complex than just reading both values independently.
> > >
> > > 1) Would be nice to test on the real HW to confirm it doesn't accept 64-bit IO.
> > 
> > Just tested, it works. No error on the memory bus. And checked assembly
> > generated was a single 64-bit instructions.
> > 
> > It might not work on other hardware revisions though. I can't remember
> > if memory bus is changing across them.
> > 
> > > 2) Still I see a benefit from using lo_hi_readq() and friends directly.
> > 
> > So it is:
> > 
> > 	u32 r0 = readl(base_plls + pll->reg64);
> > 	u32 r1 = readl(base_plls + pll->reg64 + sizeof(r0));
> > 
> > vs:
> > 
> > 	u64 r = lo_hi_readq(base_plls + pll->regs64);
>
> > 	u32 r0 = r;
> > 	u32 r1 = r >> 32;
>
> It depends to the semantics of these two. How hard do they coupled to each
> other semantically? I.o.w. can they always be considered as 64-bit register
> with the respective bitfields? (And note FIELD_GET() here is your friend.)

OLB (the memory region) has always been described as a list of 32-bit
registers. The semantics lean in the camp of two readl().

> > One is straight forward, the other uses an obscure helper that code
> > readers must understand and follows that with bit manipulation.
>
> [...]
>
> > There are two errors to handle, that makes a mess out of the code.
> > Having a little bit of repetition but straight forward code is nicer in
> > my opinion. At least we tried!
>
> Yes! Perhaps you can add a couple of words into commit message to explain
> this detail of implementation (that code in two parts is not so identical
> to be easily deduplicated).

Yes, will do. I get why from a reader's point-of-view it looks like
duplicate code.

Thanks,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com