Date:	Mon, 09 Mar 2015 10:05:04 +0100
From:	Stefan Agner <stefan@...er.ch>
To:	Bill Pringlemeir <bpringlemeir@...ps.com>,
	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	geert@...ux-m68k.org, dwmw2@...radead.org,
	computersforpeace@...il.com, mark.rutland@....com,
	boris.brezillon@...e-electrons.com, aaron@...tycactus.com,
	marb@...at.de, pawel.moll@....com, ijc+devicetree@...lion.org.uk,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	robh+dt@...nel.org, linux-mtd@...ts.infradead.org,
	kernel@...gutronix.de, galak@...eaurora.org, shawn.guo@...aro.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/5] mtd: nand: vf610_nfc: Freescale NFC for VF610, MPC5125 and others

On 2015-03-06 16:32, Bill Pringlemeir wrote:
> On  6 Mar 2015, stefan@...er.ch wrote:
> 
>> On 2015-03-06 07:15, Sascha Hauer wrote:
>>> Hi Stefan,
> 
>>> On Thu, Mar 05, 2015 at 12:10:20AM +0100, Stefan Agner wrote:
>>>> +
>>>> +static int vf610_nfc_probe_dt(struct device *dev, struct
>>>> vf610_nfc_config *cfg)
>>>> +{
>>>> +	struct device_node *np = dev->of_node;
>>>> +	int buswidth;
>>>> +	u32 clkrate;
>>>> +
>>>> +	if (!np)
>>>> +		return 1;
>>>> +
>>>> +	cfg->flash_bbt = of_get_nand_on_flash_bbt(np);
>>>> +
>>>> +	if (!of_property_read_u32(np, "clock-frequency", &clkrate))
>>>> +		cfg->clkrate = clkrate;
> 
>>> Normally the clock-frequency property tells the driver at which
>>> frequency the device actually is running, not to tell the driver at
>>> which frequency the device *should* run. It's strange to use the
>>> value of the clock-frequency property as input to
>>> clk_set_rate(). Maybe the assigned clock binding is more appropriate
>>> here, see Documentation/devicetree/bindings/clock/clock-bindings.txt.
> 
>> What we try to do here is to specify the hardware limitations. There
>> seem to be some hardware restrictions when it comes to clock
>> frequencies. There has been a rather long discussion over at
>> Freescales community about it:
>> https://community.freescale.com/thread/317074
> 
>> Not sure if this is the right way to specify the supported
>> frequencies, or should we create a custom property for this, something
>> like fsl,max-nfc-frequency = <33000000>?
> 
> On most SOC's with this controller, the input clock to the controller
> affects the NAND flash timing and other factors; so you will want to set
> it based on the board/NAND stuffed.  The clock is for synchronous logic
> in the controller and affects many properties.  
> 
> I guess Sascha's point is, the board's DT should just have some
> '&nfc_clk' node and not have this part of the driver?  Either way works.
> However, this clock is very important to get the driver to function.  It
> seem better for a user/porter to have this info in the node.  I guess
> you need to be trained to look at every node in the sub-tree for a
> device.  I think the other way might be better or a sub-system
> maintainer.  I looked at 'i2c' and other node which have a
> 'clock-frequency' parameter. 
> 
> In the Documentation/devicetree/bindings/clock/clock-bindings.txt,
> 
>     uart@...0 {
>         compatible = "fsl,imx-uart";
>         reg = <0xa000 0x1000>;
>         interrupts = <33>;
>         clocks = <&osc 0>, <&pll 1>;
>         clock-names = "baud", "register";
>     };
> 
> Here, this uart clock may affect the maximum baud rate supported by the
> device.  For this controller (vf610_nfc), the clock is like setting the
> 'baud rate'; it affect the NAND memory cycles.  There is not really any
> 'wait state' type logic in the controller register set that would allow
> the driver to work with a 'given clock' rate.  For certain a board
> should set this clock for the NAND chips they wish to support.
> 
> What would the board file look like to use clock node?
> 
> [generic]
>         nfc: nand@...e0000 {
>                 compatible = "fsl,vf610-nfc";
>                 reg = <0x400e0000 0x4000>;
>                 clocks = <&clks VF610_CLK_NFC>;
>                 clock-names = "nfc_clk";
>                 status = "disabled";
>         };
> 
> [board]
> 
> &nfc {
>         nand-bus-width = <16>;
>         nand-ecc-mode = "hw";
>         nand-on-flash-bbt;
>         nand-ecc-strength = <24>;
>         nand-ecc-step-size = <2048>;
>         pinctrl-names = "default";
>         pinctrl-0 = <&pinctrl_nfc_1>;
>         status = "okay";
>         &nfc_clk { frequency = <33000000>}  /* Like this? */

There is a property called "clock-frequency", but this is really for
static clocks (e.g. external oscillators). I don't think this is the
right approach.

> };
> 
> I don't know how to do the 'Like this?' part.  I don't think the
> 'clock-bindings.txt' explains it.  I see this is better as the the
> driver needs no 'clock handling' code.  It is definitely a little more
> obtuse for the users.

There are drivers which use the clock-frequency property to actually
specify maximum operating frequencies:

Documentation/devicetree/bindings/i2c/i2c-efm32.txt: - clock-frequency :
maximal I2C bus clock frequency in Hz.
Documentation/devicetree/bindings/media/samsung-fimc.txt:-
clock-frequency: maximum FIMC local clock (LCLK) frequency;

In the MMC subsystem there is a property called max-frequency, the SPI
subsystem uses the property spi-max-frequency to specify maximum
operating frequencies of SPI slaves.

How about just max-frequency? Or with vendor prefix, e.g.
fsl,max-frequency...?

> 
> [snip]
> 
>>> Does this driver work without device tree or not? Currently the
>>> driver bails out when device tree support is enabled but no device
>>> node is given. When device tree support is disabled in the kernel
>>> though the driver happily continues here.
> 
>> Hm, I never tried using this Driver without DT.
> 
> [snip]
> 
> I also didn't test this.  The driver was ported from Linux versions
> where DT did not exist.  It is used in some OpenWRT/68k/coldfire
> (patched) kernels and I wanted it to be useful for them.  However, we
> could probably remove the 'platform support'.  Other people are using
> this on PPC platforms and they will also have dt/of.
> 
> Currently the platform control has no way to 'pass data', so the driver
> only works with whatever defaults it has (or that is my belief).  For
> instance, those OpenWRT kernels have a 'machine file' which will set the
> 'clock-frequency' and other parameters.  We could remove the platform
> support completely if it is misleading.  I guess the KConfig would need
> a 'depends on CONFIG_OF'.
> 
> Thanks for the review,
> Bill Pringlemeir.

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