Re: [PATCH 1/5] mtd: nand: vf610_nfc: Freescale NFC for VF610, MPC5125 and others

From: Stefan Agner
Date: Mon Mar 09 2015 - 05:05:27 EST

On 2015-03-06 16:32, Bill Pringlemeir wrote:
> On 6 Mar 2015, stefan@xxxxxxxx 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:
>> 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@a000 {
> 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@400e0000 {
> 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.
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.

> [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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