Re: [PATCH v2 1/3] clk: analogbits: add Wide-Range PLL library

[Paul Walmsley]

Hi Stephen,


On 10/25/18 12:47 AM, Stephen Boyd wrote:
> Quoting Paul Walmsley (2018-10-20 06:50:22)

>> Cc: Wesley Terpstra <wesley@xxxxxxxxxx>
>> Cc: Palmer Dabbelt <palmer@xxxxxxxxxx>
>> Cc: Michael Turquette <mturquette@xxxxxxxxxxxx>
>> Cc: Stephen Boyd <sboyd@xxxxxxxxxx>
>> Cc: Megan Wachs <megan@xxxxxxxxxx>                                                                                                                    Cc: linux-clk@xxxxxxxxxxxxxxx
> Weird tab ^?


Thanks. I must be getting senile to have missed that.


>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Signed-off-by: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
>> Signed-off-by: Paul Walmsley <paul@xxxxxxxxx>
>> diff --git a/drivers/clk/analogbits/wrpll-cln28hpc.c b/drivers/clk/analogbits/wrpll-cln28hpc.c
>> new file mode 100644
>> index 000000000000..ebdef859cbf5
>> --- /dev/null
>> +++ b/drivers/clk/analogbits/wrpll-cln28hpc.c
>> @@ -0,0 +1,387 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Copyright (C) 2018 SiFive, Inc.
>> + * Wesley Terpstra
>> + * Paul Walmsley
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2 as
>> + * published by the Free Software Foundation.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> + * GNU General Public License for more details.
>> + *
> Can you drop these two paragraphs? It's duplicated from the SPDX tag.


Yep, fixed.


>> + * This library supports configuration parsing and reprogramming of
>> + * the CLN28HPC variant of the Analog Bits Wide Range PLL.  The
>> + * intention is for this library to be reusable for any device that
>> + * integrates this PLL; thus the register structure and programming
>> + * details are expected to be provided by a separate IP block driver.
>> + *
>> + * The bulk of this code is primarily useful for clock configurations
>> + * that must operate at arbitrary rates, as opposed to clock configurations
>> + * that are restricted by software or manufacturer guidance to a small,
>> + * pre-determined set of performance points.
>> + *
>> + * References:
>> + * - Analog Bits "Wide Range PLL Datasheet", version 2015.10.01
>> + * - SiFive FU540-C000 Manual v1p0, Chapter 7 "Clocking and Reset"
> Any html links?


The Analog Bits datasheet is not available on-line, to my knowledge.Â
The comments in this driver are an attempt to document it for software
developers.


The SiFive datasheet is available online; just added a link to the comments.


>> + */
>> +
>> +#include <linux/bug.h>
> Is this used?


Yes - the library doesn't compile without it:


 CC drivers/clk/analogbits/wrpll-cln28hpc.o
drivers/clk/analogbits/wrpll-cln28hpc.c: In function
'__wrpll_calc_filter_range':
drivers/clk/analogbits/wrpll-cln28hpc.c:83:3: error: implicit
declaration of function 'WARN' [-Werror=implicit-function-declaration]
ÂÂ WARN(1, "%s: post-divider reference freq out of range: %lu",
ÂÂ ^~~~

(etc.)


>> +#include <linux/err.h>
>> +#include <linux/log2.h>
>> +#include <linux/math64.h>
>> +#include <linux/clk/analogbits-wrpll-cln28hpc.h>
>> +
>> +/* MIN_INPUT_FREQ: minimum input clock frequency, in Hz (Fref_min) */
>> +#define MIN_INPUT_FREQ                 7000000
>> +
>> +/* MAX_INPUT_FREQ: maximum input clock frequency, in Hz (Fref_max) */
>> +#define MAX_INPUT_FREQ                 600000000
>> +
>> +/* MIN_POST_DIVIDE_REF_FREQ: minimum post-divider reference frequency, in Hz */
>> +#define MIN_POST_DIVR_FREQ             7000000
>> +
>> +/* MAX_POST_DIVIDE_REF_FREQ: maximum post-divider reference frequency, in Hz */
>> +#define MAX_POST_DIVR_FREQ             200000000
>> +
>> +/* MIN_VCO_FREQ: minimum VCO frequency, in Hz (Fvco_min) */
>> +#define MIN_VCO_FREQ                   2400000000UL
>> +
>> +/* MAX_VCO_FREQ: maximum VCO frequency, in Hz (Fvco_max) */
>> +#define MAX_VCO_FREQ                   4800000000ULL
>> +
>> +/* MAX_DIVQ_DIVISOR: maximum output divisor.  Selected by DIVQ = 6 */
>> +#define MAX_DIVQ_DIVISOR               64
>> +
>> +/* MAX_DIVR_DIVISOR: maximum reference divisor.  Selected by DIVR = 63 */
>> +#define MAX_DIVR_DIVISOR               64
>> +
>> +/* MAX_LOCK_US: maximum PLL lock time, in microseconds (tLOCK_max) */
>> +#define MAX_LOCK_US                    70
>> +
>> +/*
>> + * ROUND_SHIFT: number of bits to shift to avoid precision loss in the rounding
>> + *              algorithm
>> + */
>> +#define ROUND_SHIFT                    20
>> +
>> +/*
>> + * Private functions
>> + */
>> +
>> +/**
>> + * __wrpll_calc_filter_range() - determine PLL loop filter bandwidth
>> + * @post_divr_freq: input clock rate after the R divider
>> + *
>> + * Select the value to be presented to the PLL RANGE input signals, based
>> + * on the input clock frequency after the post-R-divider @post_divr_freq.
>> + * This code follows the recommendations in the PLL datasheet for filter
>> + * range selection.
>> + *
>> + * Return: The RANGE value to be presented to the PLL configuration inputs,
>> + *         or -1 upon error.
>> + */
>> +static int __wrpll_calc_filter_range(unsigned long post_divr_freq)
>> +{
>> +       u8 range;
>> +
>> +       if (post_divr_freq < MIN_POST_DIVR_FREQ ||
>> +           post_divr_freq > MAX_POST_DIVR_FREQ) {
>> +               WARN(1, "%s: post-divider reference freq out of range: %lu",
>> +                    __func__, post_divr_freq);
>> +               return -1;
>> +       }
>> +
>> +       if (post_divr_freq < 11000000)
>> +               range = 1;
>> +       else if (post_divr_freq < 18000000)
>> +               range = 2;
>> +       else if (post_divr_freq < 30000000)
>> +               range = 3;
>> +       else if (post_divr_freq < 50000000)
>> +               range = 4;
>> +       else if (post_divr_freq < 80000000)
>> +               range = 5;
>> +       else if (post_divr_freq < 130000000)
>> +               range = 6;
>> +       else
>> +               range = 7;
>> +
>> +       return range;
>> +}
>> +
>> +/**
>> + * __wrpll_calc_fbdiv() - return feedback fixed divide value
>> + * @c: ptr to a struct analogbits_wrpll_cfg record to read from
>> + *
>> + * The internal feedback path includes a fixed by-two divider; the
>> + * external feedback path does not.  Return the appropriate divider
>> + * value (2 or 1) depending on whether internal or external feedback
>> + * is enabled.  This code doesn't test for invalid configurations
>> + * (e.g. both or neither of WRPLL_FLAGS_*_FEEDBACK are set); it relies
>> + * on the caller to do so.
>> + *
>> + * Context: Any context.  Caller must protect the memory pointed to by
>> + *          @c from simultaneous modification.
> Would that ever be needed? Presumably the flag isn't changing.


It's just another way of writing that the caller needs to lock the data
pointed to by c against simultaneous modification, freeing, etc.Â
Ideally this would be obvious to callers by virtue of the pointer being
passed, but figured there was no harm in explicitly stating it. Let me
know if you want me to remove the comment.


>> + *
>> + * Return: 2 if internal feedback is enabled or 1 if external feedback
>> + *         is enabled.
>> + */
>> +static u8 __wrpll_calc_fbdiv(struct analogbits_wrpll_cfg *c)
> const c?


Done


>> +{
>> +       return (c->flags & WRPLL_FLAGS_INT_FEEDBACK_MASK) ? 2 : 1;
>> +}
>> +
>> +/**
>> + * __wrpll_calc_divq() - determine DIVQ based on target PLL output clock rate
>> + * @target_rate: target PLL output clock rate
>> + * @vco_rate: pointer to a u64 to store the computed VCO rate into
>> + *
>> + * Determine a reasonable value for the PLL Q post-divider, based on the
>> + * target output rate @target_rate for the PLL.  Along with returning the
>> + * computed Q divider value as the return value, this function stores the
>> + * desired target VCO rate into the variable pointed to by @vco_rate.
>> + *
>> + * Context: Any context.  Caller must protect the memory pointed to by
>> + *          @vco_rate from simultaneous access or modification.
>> + *
>> + * Return: a positive integer DIVQ value to be programmed into the hardware
>> + *         upon success, or 0 upon error (since 0 is an invalid DIVQ value)
>> + */
>> +static u8 __wrpll_calc_divq(u32 target_rate, u64 *vco_rate)
> Nitpick: Why so much underscore in this code?


The intention was to mark file-local functions from global functions (as
you recognize below).ÂÂ Of course, I should have just used one
underscore: just re-reviewed the C standard and turns out I misrecalled
the usual way for doing this - one underscore is appropriate. But, see
below.


>> +{
>> +       u64 s;
>> +       u8 divq = 0;
>> +
>> +       if (!vco_rate) {
>> +               WARN_ON(1);
>> +               goto wcd_out;
> Just return here, There aren't any locks so no need for goto.


(See below)


>
> Also, don't check anything? The caller is bad if this happens, and this
> is a static function so it can pretty much be found by visual
> inspection or during development?


Agreed this makes sense for the static functions. Dropped & kerneldoc
updated.


>> +       }
>> +
>> +       s = div_u64(MAX_VCO_FREQ, target_rate);
>> +       if (s <= 1) {
>> +               divq = 1;
>> +               *vco_rate = MAX_VCO_FREQ;
>> +       } else if (s > MAX_DIVQ_DIVISOR) {
>> +               divq = ilog2(MAX_DIVQ_DIVISOR);
>> +               *vco_rate = MIN_VCO_FREQ;
>> +       } else {
>> +               divq = ilog2(s);
>> +               *vco_rate = target_rate << divq;
>> +       }
>> +
>> +wcd_out:
>> +       return divq;
>> +}
>> +
>> +/**
>> + * __wrpll_update_parent_rate() - update PLL data when parent rate changes
>> + * @c: ptr to a struct analogbits_wrpll_cfg record to write PLL data to
>> + * @parent_rate: PLL input refclk rate (pre-R-divider)
>> + *
>> + * Pre-compute some data used by the PLL configuration algorithm when
>> + * the PLL's reference clock rate changes.  The intention is to avoid
>> + * computation when the parent rate remains constant - expected to be
>> + * the common case.
>> + *
>> + * Returns: 0 upon success or -1 if the reference clock rate is out of range.
>> + */
>> +static int __wrpll_update_parent_rate(struct analogbits_wrpll_cfg *c,
>> +                                     unsigned long parent_rate)
>> +{
>> +       u8 max_r_for_parent;
> Does this need to be a u8? It makes the min_t below required instead of
> using an unsigned int.


I've changed this as you request, and added the (subsequently required)
type suffix to the macro value, along with an appropriate comment.


>> +
>> +       if (parent_rate > MAX_INPUT_FREQ || parent_rate < MIN_POST_DIVR_FREQ)
>> +               return -1;
> Please pick some valid error code instead of -1 (I thought checkpatch
> complained about this?). 


checkpatch isn't complaining here. Anyway, fixed to return a reasonable
error code.


> Is this case even possible?


If analogbits_wrpll_configure_for_rate() is called with a parent_rate
outside of the PLL's specification range, it seems reasonable to fail --
or do you see the situation differently? These parent rate restrictions
are defined in the PLL datasheet.


>  It would be nicer
> if this function just returned void.


ÂIs it that you'd prefer the rate check to be moved into
analogbits_wrpll_configure_for_rate()? Or that you'd prefer that I not
validate the input parent rate at all?


>> +
>> +       c->_parent_rate = parent_rate;
>> +       max_r_for_parent = div_u64(parent_rate, MIN_POST_DIVR_FREQ);
>> +       c->_max_r = min_t(u8, MAX_DIVR_DIVISOR, max_r_for_parent);
>> +
>> +       /* Round up */
>> +       c->_init_r = div_u64(parent_rate + MAX_POST_DIVR_FREQ - 1,
>> +                            MAX_POST_DIVR_FREQ);
>> +
>> +       return 0;
>> +}
>> +
>> +/*
>> + * Public functions
>> + */
>> +
>> +/**
>> + * analogbits_wrpll_configure() - compute PLL configuration for a target rate
>> + * @c: ptr to a struct analogbits_wrpll_cfg record to write into
>> + * @target_rate: target PLL output clock rate (post-Q-divider)
>> + * @parent_rate: PLL input refclk rate (pre-R-divider)
>> + *
>> + * Given a pointer to a PLL context @c, a desired PLL target output
>> + * rate @target_rate, and a reference clock input rate @parent_rate,
>> + * compute the appropriate PLL signal configuration values.  PLL
>> + * reprogramming is not glitchless, so the caller should switch any
>> + * downstream logic to a different clock source or clock-gate it
>> + * before presenting these values to the PLL configuration signals.
>> + *
>> + * The caller must pass this function a pre-initialized struct
>> + * analogbits_wrpll_cfg record: either initialized to zero (with the
>> + * exception of the .name and .flags fields) or read from the PLL.
>> + *
>> + * Context: Any context.  Caller must protect the memory pointed to by @c
>> + *          from simultaneous access or modification.
>> + *
>> + * Return: 0 upon success; anything else upon failure.
>> + */
>> +int analogbits_wrpll_configure_for_rate(struct analogbits_wrpll_cfg *c,
>> +                                       u32 target_rate,
>> +                                       unsigned long parent_rate)
>> +{
>> +       unsigned long ratio;
>> +       u64 target_vco_rate, delta, best_delta, f_pre_div, vco, vco_pre;
>> +       u32 best_f, f, post_divr_freq, fbcfg;
>> +       u8 fbdiv, divq, best_r, r;
>> +
>> +       if (!c)
>> +               return -1;
> -EINVAL? Of course, it's probably better to just not care and blow up if
> the user of the API passes in NULL.


Agreed for statically-scoped functions it's most likely overkill. But
for globally-scoped functions like this, it seemed better to catch the
errors rather than to blow up. The underlying approach was intended to
be standard precondition-based (paranoia-based?) development. Seems
like I often screw up the calling side during development, so figured
this approach would save human time. The performance impact seems
minimal. That said, if it's your strong preference or requirement for
these to be dropped, I will of course do it - let me know.


In the meantime, have changed the return value to -EINVAL per your comment.


>> +
>> +       if (c->flags == 0) {
>> +               WARN(1, "%s called with uninitialized PLL config", __func__);
> Do we really need the stacktrace in this case? Or can this be downgraded
> to a pr_warn() and return some error value that isn't -1?


Since this case should never happen, and indicates either a bug in the
code or corrupted memory, the intention was to warn loudly so it was
difficult to be ignore. So I would prefer to keep it annoying. But
again, if it's important, would be OK with changing it - let me know.Â
Usually I try to be more paranoid with the globally-scoped functions
(like this one) than with the file-local functions.


>> +               return -1;
>> +       }
>> +
>> +       fbcfg = WRPLL_FLAGS_INT_FEEDBACK_MASK | WRPLL_FLAGS_EXT_FEEDBACK_MASK;
>> +       if ((c->flags & fbcfg) == fbcfg) {
>> +               WARN(1, "%s called with invalid PLL config", __func__);
>> +               return -1;
>> +       }
>> +
>> +       if (c->flags == WRPLL_FLAGS_EXT_FEEDBACK_MASK) {
>> +               WARN(1, "%s: external feedback mode not currently supported",
>> +                    __func__);
>> +               return -1;
>> +       }
> Like a bunch of this stuff, why are we checking things that the caller
> should know to do itself? I'd prefer we keep things simple and assume
> the caller knows what they're doing.


My thinking was that it's easy to screw up if someone is unfamiliar with
the library. Since the preconditions are computationally simple to
verify, the runtime cost is essentially zero. Put differently, they are
intended to trade off a small amount of CPU time to save a larger amount
of human time.


>> +
>> +       /* Initialize rounding data if it hasn't been initialized already */
>> +       if (parent_rate != c->_parent_rate) {
>> +               if (__wrpll_update_parent_rate(c, parent_rate)) {
>> +                       pr_err("%s: PLL input rate is out of range\n",
>> +                              __func__);
>> +                       return -1;
>> +               }
>> +       }
>> +
>> +       c->flags &= ~WRPLL_FLAGS_RESET_MASK;
>> +
>> +       /* Put the PLL into bypass if the user requests the parent clock rate */
>> +       if (target_rate == parent_rate) {
>> +               c->flags |= WRPLL_FLAGS_BYPASS_MASK;
>> +               return 0;
>> +       }
>> +       c->flags &= ~WRPLL_FLAGS_BYPASS_MASK;
>> +
>> +       /* Calculate the Q shift and target VCO rate */
>> +       divq = __wrpll_calc_divq(target_rate, &target_vco_rate);
>> +       if (divq == 0)
>> +               return -1;
>> +       c->divq = divq;
>> +
>> +       /* Precalculate the pre-Q divider target ratio */
>> +       ratio = div64_u64((target_vco_rate << ROUND_SHIFT), parent_rate);
>> +
>> +       fbdiv = __wrpll_calc_fbdiv(c);
>> +       best_r = 0;
>> +       best_f = 0;
>> +       best_delta = MAX_VCO_FREQ;
>> +
>> +       /*
>> +        * Consider all values for R which land within
>> +        * [MIN_POST_DIVR_FREQ, MAX_POST_DIVR_FREQ]; prefer smaller R
>> +        */
>> +       for (r = c->_init_r; r <= c->_max_r; ++r) {
>> +               /* What is the best F we can pick in this case? */
>> +               f_pre_div = ratio * r;
>> +               f = (f_pre_div + (1 << ROUND_SHIFT)) >> ROUND_SHIFT;
>> +               f >>= (fbdiv - 1);
>> +
>> +               post_divr_freq = div_u64(parent_rate, r);
>> +               vco_pre = fbdiv * post_divr_freq;
>> +               vco = vco_pre * f;
>> +
>> +               /* Ensure rounding didn't take us out of range */
>> +               if (vco > target_vco_rate) {
>> +                       --f;
>> +                       vco = vco_pre * f;
>> +               } else if (vco < MIN_VCO_FREQ) {
>> +                       ++f;
>> +                       vco = vco_pre * f;
>> +               }
>> +
>> +               delta = abs(target_rate - vco);
>> +               if (delta < best_delta) {
>> +                       best_delta = delta;
>> +                       best_r = r;
>> +                       best_f = f;
>> +               }
>> +       }
>> +
>> +       c->divr = best_r - 1;
>> +       c->divf = best_f - 1;
>> +
>> +       post_divr_freq = div_u64(parent_rate, best_r);
>> +
>> +       /* Pick the best PLL jitter filter */
>> +       c->range = __wrpll_calc_filter_range(post_divr_freq);
>> +
>> +       return 0;
>> +}
>> +
>> +/**
>> + * analogbits_wrpll_calc_output_rate() - calculate the PLL's target output rate
>> + * @c: ptr to a struct analogbits_wrpll_cfg record to read from
>> + * @parent_rate: PLL refclk rate
>> + *
>> + * Given a pointer to the PLL's current input configuration @c and the
>> + * PLL's input reference clock rate @parent_rate (before the R
>> + * pre-divider), calculate the PLL's output clock rate (after the Q
>> + * post-divider)
>> + *
>> + * Context: Any context.  Caller must protect the memory pointed to by @c
>> + *          from simultaneous modification.
>> + *
>> + * Return: the PLL's output clock rate, in Hz.
>> + */
>> +unsigned long analogbits_wrpll_calc_output_rate(struct analogbits_wrpll_cfg *c,
>> +                                               unsigned long parent_rate)
>> +{
>> +       u8 fbdiv;
>> +       u64 n;
>> +
>> +       WARN(c->flags & WRPLL_FLAGS_EXT_FEEDBACK_MASK,
>> +            "external feedback mode not yet supported");
>> +
>> +       fbdiv = __wrpll_calc_fbdiv(c);
>> +       n = parent_rate * fbdiv * (c->divf + 1);
>> +       n = div_u64(n, (c->divr + 1));
>> +       n >>= c->divq;
>> +
>> +       return n;
>> +}
>> +
>> +/**
>> + * analogbits_wrpll_calc_max_lock_us() - return the time for the PLL to lock
>> + * @c: ptr to a struct analogbits_wrpll_cfg record to read from
>> + *
>> + * Return the minimum amount of time (in microseconds) that the caller
>> + * must wait after reprogramming the PLL to ensure that it is locked
>> + * to the input frequency and stable.  This is likely to depend on the DIVR
>> + * value; this is under discussion with the manufacturer.
>> + *
>> + * Return: the minimum amount of time the caller must wait for the PLL
>> + *         to lock (in microseconds)
>> + */
>> +unsigned int analogbits_wrpll_calc_max_lock_us(struct analogbits_wrpll_cfg *c)
> const c?


Done


>> +{
>> +       return MAX_LOCK_US;
> And we're planning to change this in the future?


Yes, we're in a discussion with the PLL designer about this, and I
expect it to change within another merge window.


>> +}
>> diff --git a/include/linux/clk/analogbits-wrpll-cln28hpc.h b/include/linux/clk/analogbits-wrpll-cln28hpc.h
>> new file mode 100644
>> index 000000000000..cc4268f16067
>> --- /dev/null
>> +++ b/include/linux/clk/analogbits-wrpll-cln28hpc.h
>> @@ -0,0 +1,99 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * Copyright (C) 2018 SiFive, Inc.
>> + * Wesley Terpstra
>> + * Paul Walmsley
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2 as
>> + * published by the Free Software Foundation.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> + * GNU General Public License for more details.
>> + */
>> +
>> +#ifndef __LINUX_CLK_ANALOGBITS_WRPLL_CLN28HPC_H
>> +#define __LINUX_CLK_ANALOGBITS_WRPLL_CLN28HPC_H
>> +
>> +#include <linux/types.h>
>> +
>> +/* DIVQ_VALUES: number of valid DIVQ values */
>> +#define DIVQ_VALUES                            6
>> +
>> +/*
>> + * Bit definitions for struct analogbits_wrpll_cfg.flags
>> + *
>> + * WRPLL_FLAGS_BYPASS_FLAG: if set, the PLL is either in bypass, or should be
>> + *     programmed to enter bypass
>> + * WRPLL_FLAGS_RESET_FLAG: if set, the PLL is in reset
>> + * WRPLL_FLAGS_INT_FEEDBACK_FLAG: if set, the PLL is configured for internal
>> + *     feedback mode
>> + * WRPLL_FLAGS_EXT_FEEDBACK_FLAG: if set, the PLL is configured for external
>> + *     feedback mode (not yet supported by this driver)
>> + *
>> + * The flags WRPLL_FLAGS_INT_FEEDBACK_FLAG and WRPLL_FLAGS_EXT_FEEDBACK_FLAG are
>> + * mutually exclusive.  If both bits are set, or both are zero, the struct
>> + * analogbits_wrpll_cfg record is uninitialized or corrupt.
>> + */
>> +#define WRPLL_FLAGS_BYPASS_SHIFT               0
>> +#define WRPLL_FLAGS_BYPASS_MASK                BIT(WRPLL_FLAGS_BYPASS_SHIFT)
>> +#define WRPLL_FLAGS_RESET_SHIFT                1
>> +#define WRPLL_FLAGS_RESET_MASK         BIT(WRPLL_FLAGS_RESET_SHIFT)
>> +#define WRPLL_FLAGS_INT_FEEDBACK_SHIFT 2
>> +#define WRPLL_FLAGS_INT_FEEDBACK_MASK  BIT(WRPLL_FLAGS_INT_FEEDBACK_SHIFT)
>> +#define WRPLL_FLAGS_EXT_FEEDBACK_SHIFT 3
>> +#define WRPLL_FLAGS_EXT_FEEDBACK_MASK  BIT(WRPLL_FLAGS_EXT_FEEDBACK_SHIFT)
>> +
>> +/**
>> + * struct analogbits_wrpll_cfg - WRPLL configuration values
>> + * @divr: reference divider value (6 bits), as presented to the PLL signals.
>> + * @divf: feedback divider value (9 bits), as presented to the PLL signals.
>> + * @divq: output divider value (3 bits), as presented to the PLL signals.
>> + * @flags: PLL configuration flags.  See above for more information.
>> + * @range: PLL loop filter range.  See below for more information.
>> + * @_output_rate_cache: cached output rates, swept across DIVQ.
>> + * @_parent_rate: PLL refclk rate for which values are valid
>> + * @_max_r: maximum possible R divider value, given @parent_rate
>> + * @_init_r: initial R divider value to start the search from
>> + *
>> + * @divr, @divq, @divq, @range represent what the PLL expects to see
>> + * on its input signals.  Thus @divr and @divf are the actual divisors
>> + * minus one.  @divq is a power-of-two divider; for example, 1 =
>> + * divide-by-2 and 6 = divide-by-64.  0 is an invalid @divq value.
>> + *
>> + * When initially passing a struct analogbits_wrpll_cfg record, the
>> + * record should be zero-initialized with the exception of the @flags
>> + * field.  The only flag bits that need to be set are either
>> + * WRPLL_FLAGS_INT_FEEDBACK or WRPLL_FLAGS_EXT_FEEDBACK.
>> + *
>> + * Field names beginning with an underscore should be considered
>> + * private to the wrpll-cln28hpc.c code.
> Ah ok. Are the functions also done this way for a private/public API
> split. 


Yep.


> We don't typically care about this in the kernel, just mark it
> static if it's private and non-static if it's public for functions.


Used to do it this way a long ago, perhaps a bit less incompetently -
here's an example:


https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/mach-omap2/omap_hwmod.c?h=v4.1#n383


I personally prefer the explicit indication that the identifier was
scoped file-local in the calling functions. But am fine changing it per
your comments, and have done so.


> Underscores just make lines longer. 


Hehe.


> And for struct members we have
> kernel doc 'private' tag that can tell users to not do something. Or
> worst case, an opaque pointer is stored and only defined in the C file.


In any case, I've changed the identifier names to align to your comments.


>> + */
>> +struct analogbits_wrpll_cfg {
>> +       u8 divr;
>> +       u8 divq;
>> +       u8 range;
>> +       u8 flags;
>> +       u16 divf;
>> +       u32 _output_rate_cache[DIVQ_VALUES];
>> +       unsigned long _parent_rate;
>> +       u8 _max_r;
>> +       u8 _init_r;
>> +};
>> +
>> +/*
>> + * Function prototypes
>> + */
> Please don't have these types of comments. They don't help.


Dropped this one. Do you want me to nuke the similar comments in
wrpll-cln28hpc.c, separating the private function section from the
public functions, for example?


Thanks for your review,


- Paul