linux-kernel - Re: [PATCH v8 2/3] phy: zynqmp: Add PHY driver for the Xilinx ZynqMP Gigabit Transceiver

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Message-ID: <20200624211458.GA29023@pendragon.ideasonboard.com>
Date:   Thu, 25 Jun 2020 00:14:58 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Kishon Vijay Abraham I <kishon@...com>,
        Anurag Kumar Vulisha <anurag.kumar.vulisha@...inx.com>,
        Michal Simek <michal.simek@...inx.com>
Subject: Re: [PATCH v8 2/3] phy: zynqmp: Add PHY driver for the Xilinx ZynqMP
 Gigabit Transceiver

Hi Vinod,

On Wed, Jun 24, 2020 at 10:56:35PM +0530, Vinod Koul wrote:
> On 24-06-20, 19:39, Laurent Pinchart wrote:
> 
> > > > +/* Number of GT lanes */
> > > > +#define NUM_LANES			4
> > > 
> > > Should this be coded in driver like this? Maybe future versions of
> > > hardware will have more lanes..? Why not describe this in DT?
> > 
> > This macro is used to avoid hardcoding 4 in the driver, to make sure
> > that all the code that deal with the number of lanes use a consistent
> > value and is readable. There's no foreseen new version of the IP that
> > would have more lane, so I don't think this should go in DT. Otherwise
> > we'd have to encode there pretty much any parameter that could one day
> > possibly change in a different universe :-)
> > 
> > Let's also note that even when parameters change between IP versions, it
> > doesn't always make sense to specify them explicitly in DT. It's totally
> > fine to have a table of parameter values in the driver, indexed by
> > compatible string. Whether to set a parameter explicitly in DT or handle
> > it in the driver usually depends on how frequently that parameter can
> > change, if it can vary between different integrations of the same IP
> > version, ...
> > 
> > In this specific case, as there's no foreseen change, we can't really
> > tell how it would change if it did one day. I thus think hardcoding the
> > parameter in the driver is fine, and in the worst case, we can add a
> > parameter in DT later and default to 4 if not specified. Same reasoning
> > for CONTROLLERS_PER_LANE.
> 
> yeah not every parameter can be coded and we should use compatible as
> well, but I would disagree with no future revision planned. It will
> happen not now, but sometime in year or so :) Been around devices has
> taught me that only constant thing is change in hardware!

I don't dispute it will not happen, my point is that, without a new
revision planned, we don't have enough information to tell which option
would be best to support future revisions. As I know we won't be blocked
(both compat string matching and adding new optional properties with
appropriate defaults will be viable options), I'm not concerned.

> Yes but this is not a deal breaker atm, will leave upto you
> 
> > > > +
> > > > +/* SIOU SATA control register */
> > > > +#define SATA_CONTROL_OFFSET		0x0100
> > > > +
> > > > +/* Total number of controllers */
> > > > +#define CONTROLLERS_PER_LANE		5
> > > 
> > > Same question for this as well..
> > > 
> > > > +/*
> > > > + * I/O Accessors
> > > > + */
> > > > +
> > > > +static inline u32 xpsgtr_read(struct xpsgtr_dev *gtr_dev, u32 reg)
> > > > +{
> > > > +	return readl(gtr_dev->serdes + reg);
> > > > +}
> > > > +
> > > > +static inline void xpsgtr_write(struct xpsgtr_dev *gtr_dev, u32 reg, u32 value)
> > > > +{
> > > > +	writel(value, gtr_dev->serdes + reg);
> > > > +}
> > > > +
> > > > +static inline void xpsgtr_clr_set(struct xpsgtr_dev *gtr_dev, u32 reg,
> > > > +				  u32 clr, u32 set)
> > > 
> > > wouldn't it be apt to rename this to xpsgtr_modify() and with args as
> > > value and mask, somehow I find that more simpler...
> > 
> > "modify" sounds more vague to me. I've also kept xpsgtr_clr_set() as
> > that's what the original author used.
> 
> yeah maybe that was a wrong choice of term, I guess update looks best.
> We really are missing update api in kernel! I see regmap does provide
> _update_bits() api
> 
> > > Also, please align second line with opening brace of preceding line
> > 
> > It is aligned, the first line is affected by the + and > in the mail,
> > while the second line uses tabs and thus isn't.
> 
> ok

-- 
Regards,

Laurent Pinchart