Re: [PATCH v2 2/5] mfd: lochnagar: Add support for the Cirrus Logic Lochnagar

[Lee Jones]

On Thu, 25 Oct 2018, Charles Keepax wrote:

> On Thu, Oct 25, 2018 at 12:42:05PM +0100, Lee Jones wrote:
> > On Thu, 25 Oct 2018, Richard Fitzgerald wrote:
> > > On 25/10/18 09:26, Charles Keepax wrote:
> > > > On Thu, Oct 25, 2018 at 08:44:59AM +0100, Lee Jones wrote:
> > > > > On Mon, 08 Oct 2018, Charles Keepax wrote:
> > > > > > From: Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
> > > > > > +static const struct reg_default lochnagar1_reg_defaults[] = {
> > > > > > +	{ LOCHNAGAR1_CDC_AIF1_SEL,    0x00 },
> > > > > > +	{ LOCHNAGAR1_CDC_AIF2_SEL,    0x00 },
> > > > ...
> > > > > > +	{ LOCHNAGAR1_LED1,            0x00 },
> > > > > > +	{ LOCHNAGAR1_LED2,            0x00 },
> > > > > > +	{ LOCHNAGAR1_I2C_CTRL,        0x01 },
> > > > > > +};
> > > > > 
> > > > > Why do you need to specify each register value?
> > > > 
> > > > The way regmap operates it needs to know the starting value of
> > > > each register. It will use this to initialise the cache and to
> > > > determine if writes need to actually update the hardware on
> > > > cache_syncs after devices have been powered back up.
> > 
> > That sounds crazy to me.  Some devices have thousands of registers.
> > At a line per register, that's thousands of lines of code/cruft.
> > Especially seeing as most (sane?) register layouts I've seen default
> > to zero.  Then default values can be changed at the leisure of the
> > s/w.
> > 
> > Even if it is absolutely critical that you have to supply these to
> > Regmap up-front, instead of on first use/read, why can't you just
> > supply the oddball non-zero ones?
> 
> I don't think you can sensibly get away with not supplying
> default values. You say most sane register layouts have zero
> values, alas again you may not be the biggest fan of our hardware
> guys. The Lochnagar actually does have mostly zero defaults but
> that is very much not generally the case with our hardware.
> 
> One of the main aims of regmap is to avoid bus accesses when
> possible but I guess on the first write/read to any register you
> could insert a read to pull the default, although I do worry
> there is some corner case/flexibility that is going to cause
> headaches later.

This is basically what I am suggesting.

> I am not sure that dynamically constructing the defaults would be
> possible from a table of non-zero ones, or at least doing so would
> probably require a fairly major change to the way registers are
> specified since with the current callback based approach and a
> sparse regmap you could need to iterate over billions of
> potential registers to build the table.

What if a valid register range was provided with only the non-zero
values?

> > > > > > +static bool lochnagar2_readable_register(struct device *dev, unsigned int reg)
> > > > > > +{
> > > > > > +	switch (reg) {
> > > > > > +	case LOCHNAGAR_SOFTWARE_RESET:
> > > > > > +	case LOCHNAGAR_FIRMWARE_ID1:
> > > > > > +	case LOCHNAGAR_FIRMWARE_ID2:
> > > > ...
> > > > > > +	case LOCHNAGAR2_MICVDD_CTRL2:
> > > > > > +	case LOCHNAGAR2_VDDCORE_CDC_CTRL1:
> > > > > > +	case LOCHNAGAR2_VDDCORE_CDC_CTRL2:
> > > > > > +	case LOCHNAGAR2_SOUNDCARD_AIF_CTRL:
> > > > > > +		return true;
> > > > > > +	default:
> > > > > > +		return false;
> > > > > > +	}
> > > > > > +}
> > > > > > +
> > > > > > +static bool lochnagar2_volatile_register(struct device *dev, unsigned int reg)
> > > > > > +{
> > > > > > +	switch (reg) {
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL1:
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL2:
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL3:
> > > > ...
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL13:
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL14:
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL15:
> > > > > > +	case LOCHNAGAR2_GPIO_CHANNEL16:
> > > > > > +	case LOCHNAGAR2_ANALOGUE_PATH_CTRL1:
> > > > > > +		return true;
> > > > > > +	default:
> > > > > > +		return false;
> > > > > > +	}
> > > > > > +}
> > > > > 
> > > > > This is getting silly now.  Can't you use ranges?
> > > > 
> > > > I can if you feel strongly about it? But it does make the drivers
> > > > much more error prone and significantly more annoying to work
> > > > with. I find it is really common to be checking that a register
> > > > is handled correctly through the regmap callbacks and it is nice
> > > > to just be able to grep for that. Obviously this won't work for
> > > > all devices/regmaps as well since many will not have consecutive
> > > > addresses on registers, for example having multi-byte registers
> > > > that are byte addressed.
> > > > 
> > > > How far would you like me to take this as well? Is it just the
> > > > numeric registers you want ranges for ie.
> > > > 
> > > > LOCHNAGAR2_GPIO_CHANNEL1...LOCHNAGAR_GPIO_CHANNEL16
> > > > 
> > > > Or is it all consecutive registers even if they are unrelated
> > > > (exmaple is probably not accurate as I haven't checked the
> > > > addresses):
> > > > 
> > > > LOCHNAGAR2_GPIO_CHANNEL1...LOCHNAGAR2_ANALOGURE_PATH_CTRL1
> > > > 
> > > > I don't mind the first at all but the second is getting really
> > > > horrible in my opinion.
> > 
> > My issue is that we have one end of the scale, where contributors are
> > submitting patches, trying to remove a line of code here, a line of
> > code there, then there are patches like this one which describe the
> > initial value, readable status, writable status and volatile status of
> > each and every register.
> 
> Removing code is one thing but I would argue that data tables are
> somewhat less of a concern. I guess kernel size for very small
> systems but then is someone going to be using Lochnagar on one of
> those.
> 
> > The API is obscene and needs a re-work IMHO.
> > 
> > I really hope we do not really have to list every register, but *if we
> > absolutely must*, let's do it once:
> > 
> >   REG_ADDRESS, WRV, INITIAL_VALUE
> 
> It might be possible to combine all these into one thing,
> although again its a fairly major core change.

I'm not suggesting that this will be solved overnight.

> > > > > > +static const struct reg_default lochnagar2_reg_defaults[] = {
> > > > > > +	{ LOCHNAGAR2_CDC_AIF1_CTRL,         0x0000 },
> > > > > > +	{ LOCHNAGAR2_CDC_AIF2_CTRL,         0x0000 },
> > > > > > +	{ LOCHNAGAR2_CDC_AIF3_CTRL,         0x0000 },
> > > > > > +	{ LOCHNAGAR2_DSP_AIF1_CTRL,         0x0000 },
> > > > ...
> > > > > > +	{ LOCHNAGAR2_MINICARD_RESETS,       0x0000 },
> > > > > > +	{ LOCHNAGAR2_ANALOGUE_PATH_CTRL2,   0x0000 },
> > > > > > +	{ LOCHNAGAR2_COMMS_CTRL4,           0x0001 },
> > > > > > +	{ LOCHNAGAR2_SPDIF_CTRL,            0x0008 },
> > > > > > +	{ LOCHNAGAR2_POWER_CTRL,            0x0001 },
> > > > > > +	{ LOCHNAGAR2_SOUNDCARD_AIF_CTRL,    0x0000 },
> > > > > > +};
> > > > > 
> > > > > OMG!  Vile, vile vile!
> > > > 
> > > > I really feel this isn't the driver you are objecting to as such
> > > > but the way regmap operates and also we seem to always have the same
> > > > discussions around regmap every time we push a driver.
> > 
> > Absolutely.  I didn't like it before.  I like it even less now.
> 
> I guess the question from my side becomes do you want to block
> this driver pending on major refactoring to regmap? I will have a
> think about what I can do but its going to affect a LOT of drivers.

No one is NACKing this driver.

We're using it as a vehicle for discussion.

> > > > > > +	ret = devm_of_platform_populate(dev);
> > > > > > +	if (ret < 0) {
> > > > > > +		dev_err(dev, "Failed to populate child nodes: %d\n", ret);
> > > > > > +		return ret;
> > > > > > +	}
> > > > > 
> > > > > Please do not mix OF and MFD device registration strategies.
> > > > > 
> > > > > Pick one and register all devices through your chosen method.
> > > > 
> > > > Hmmm we use this to do things like register some fixed regulators
> > > > and clocks that don't need any control but do need to be associated
> > > > with the device. I could do that through the MFD although it is in
> > > > direct conflict with the feedback on the clock patches I received
> > > > to move the fixed clocks into devicetree rather than registering
> > > > them manually (see v2 of the patch chain).
> > 
> > The I suggest moving everything to DT.
> 
> I will have a look and see what that would look like.

Thanks.

-- 
Lee Jones [æçæ]
Linaro Services Technical Lead
Linaro.org â Open source software for ARM SoCs
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