linux-kernel - Re: [PATCH 1/2] pinctrl: bindings: pinctrl: Add support for TI's IODelay configuration

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Message-ID: <20150310153321.GO5264@atomide.com>
Date:	Tue, 10 Mar 2015 08:33:21 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Nishanth Menon <nm@...com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Linux-OMAP <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Lokesh Vutla <lokeshvutla@...com>
Subject: Re: [PATCH 1/2] pinctrl: bindings: pinctrl: Add support for TI's
 IODelay configuration

* Linus Walleij <linus.walleij@...aro.org> [150310 03:39]:
> On Wed, Mar 4, 2015 at 1:00 AM, Nishanth Menon <nm@...com> wrote:
> 
> > +Configuration definition follows similar model as the pinctrl-single:
> > +The groups of pin configuration are defined under "pinctrl-single,pins"
> > +
> > +&dra7_iodelay_core {
> > +       mmc2_iodelay_3v3_conf: mmc2_iodelay_3v3_conf {
> > +               pinctrl-single,pins = <
> > +                       0x18c (A_DELAY(0) | G_DELAY(120))       /* CFG_GPMC_A19_IN */
> > +                       0x1a4 (A_DELAY(265) | G_DELAY(360))     /* CFG_GPMC_A20_IN */
> > +                       0x1b0 (A_DELAY(0) | G_DELAY(120))       /* CFG_GPMC_A21_IN */
> > +                       0x1bc (A_DELAY(0) | G_DELAY(120))       /* CFG_GPMC_A22_IN */
> > +                       0x1c8 (A_DELAY(287) | G_DELAY(420))     /* CFG_GPMC_A23_IN */
> > +                       0x1d4 (A_DELAY(144) | G_DELAY(240))     /* CFG_GPMC_A24_IN */
> > +                       0x1e0 (A_DELAY(0) | G_DELAY(0))         /* CFG_GPMC_A25_IN */
> > +                       0x1ec (A_DELAY(120) | G_DELAY(0))       /* CFG_GPMC_A26_IN */
> > +                       0x1f8 (A_DELAY(120) | G_DELAY(180))     /* CFG_GPMC_A27_IN */
> > +                       0x360 (A_DELAY(0) | G_DELAY(0))         /* CFG_GPMC_CS1_IN */
> > +               >;
> > +       };
> > +};
> 
> But wait.
> 
> The promise when we merged pinctrl-single was that this driver was to be used
> when the system had a one-register-per-pin layout and it was easy to do device
> trees based on that.
> 
> We were very reluctant to accept that even though we didn't even have the
> generic pin control bindings in place, the argument being that the driver
> should know the detailed register layout, it should not be described in the
> device tree. We eventually caved in and accepted it as an exception.

Hey let's get few things straight here. There's nothing stopping the
driver from knowing a detailed register layout with the pinctrl-single
binding. This can be very easily done based on the compatible flag and
match data as needed and check the values provided.

And let's also recap the reasons for the pinctrl-single binding. The
the main reason for the pinctrl-single binding is to avoid mapping
individual register bits to device tree compatible string properties.

Imagine doing that for hundreds of similar registers. Believe me, I tried
using device tree properties initially and it just sucked big time. For
larger amounts of dts data, it's best to stick to numeric values and
phandles in the device tree data and rely on the preprocessor for
getting the values right.

Finally, we don't want to build databases into the kernel drivers for
every SoC. We certainly have plenty fo those already.
 
> Since this pin controller is not using pinctrl-single it does not enjoy that
> privilege and you need to explain why this pin controller cannot use the
> generic bindings like so many other pin controllers have since started
> to do. ("It is in the same SoC" is not an acceptable argument.)
>
> What is wrong with this:
> Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

Nishanth, care to explain your reasons for using pinctrl-single binding
here vs the generic binding? Is the amount of string parsing with the
data an issue here?
 
> We can add generic delay bindings to the list, it even seems like
> a good idea to do so, as it is likely something that will come up in
> other hardware and will be needed for ACPI etc going forward.

We certainly need to make setting delays (with values) generic, no
doubt about that.

If the large amount of data is not an issue here, we could maybe set
each iodelay register as a separate device? Then the binding could
be just along the interrupts-extended type binding:

foo = <&bar 0x18c A_DELAY(0) G_DELAY(120)>;

Where the 0x18c is the instance offset of the register within the
ti,dra7-iodelay compatible controller.

Regards,

Tony
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