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

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Message-ID: <ZeCnZ0Py62EyKI9Z@smile.fi.intel.com>
Date: Thu, 29 Feb 2024 17:48:55 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Théo Lebrun <theo.lebrun@...tlin.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()

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.)

> 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).

-- 
With Best Regards,
Andy Shevchenko