linux/drivers/pwm/pwm-berlin.c
Thomas Hebb 4de445cb43 pwm: berlin: Don't use broken prescaler values
The Berlin PWM driver is currently broken on at least BG2CD. The
symptoms manifest as a very non-linear and erratic mapping from the duty
cycle configured in software to the duty cycle produced by hardware.

The cause of the bug is software's configuration of the prescaler, and
in particular its usage of the six prescaler values between the minimum
value of 1 and the maximum value of 4096. As it turns out, these six
values do not actually slow down the PWM clock; rather, they emulate
slowing down the clock by internally multiplying the value of TCNT.

This would be a fine trick, if not for the fact that the internal,
scaled TCNT value has no extra bits beyond the 16 already exposed to
software in the register. What this means is that, for a prescaler of 4,
the software must ensure that the top two bits of TCNT are not set,
because hardware will chop them off; for a prescaler of 8, the top three
bits must not be set, and so forth. Software does not currently ensure
this, resulting in a TCNT several orders of magnitude lower than
intended any time one of those six prescalers are selected.

Because hardware chops off the high bits in its internal shift, the
middle six prescalers don't actually allow *anything* that the first
doesn't. In fact, they are strictly worse than the first, since the
internal shift of TCNT prevents software from setting the low bits,
decreasing the resolution, without providing any extra high bits.

By skipping the useless prescalers entirely, this patch both fixes the
driver's behavior and increases its performance (since, when the 4096
prescaler is selected, it now does only a single shift rather than the
seven successive divisions it did before).

Tested on BG2CD.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
2018-07-09 18:57:03 +02:00

308 lines
7.8 KiB
C

/*
* Marvell Berlin PWM driver
*
* Copyright (C) 2015 Marvell Technology Group Ltd.
*
* Author: Antoine Tenart <antoine.tenart@free-electrons.com>
*
* This file is licensed under the terms of the GNU General Public
* License version 2. This program is licensed "as is" without any
* warranty of any kind, whether express or implied.
*/
#include <linux/clk.h>
#include <linux/io.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/platform_device.h>
#include <linux/pwm.h>
#include <linux/slab.h>
#define BERLIN_PWM_EN 0x0
#define BERLIN_PWM_ENABLE BIT(0)
#define BERLIN_PWM_CONTROL 0x4
/*
* The prescaler claims to support 8 different moduli, configured using the
* low three bits of PWM_CONTROL. (Sequentially, they are 1, 4, 8, 16, 64,
* 256, 1024, and 4096.) However, the moduli from 4 to 1024 appear to be
* implemented by internally shifting TCNT left without adding additional
* bits. So, the max TCNT that actually works for a modulus of 4 is 0x3fff;
* for 8, 0x1fff; and so on. This means that those moduli are entirely
* useless, as we could just do the shift ourselves. The 4096 modulus is
* implemented with a real prescaler, so we do use that, but we treat it
* as a flag instead of pretending the modulus is actually configurable.
*/
#define BERLIN_PWM_PRESCALE_4096 0x7
#define BERLIN_PWM_INVERT_POLARITY BIT(3)
#define BERLIN_PWM_DUTY 0x8
#define BERLIN_PWM_TCNT 0xc
#define BERLIN_PWM_MAX_TCNT 65535
struct berlin_pwm_channel {
u32 enable;
u32 ctrl;
u32 duty;
u32 tcnt;
};
struct berlin_pwm_chip {
struct pwm_chip chip;
struct clk *clk;
void __iomem *base;
};
static inline struct berlin_pwm_chip *to_berlin_pwm_chip(struct pwm_chip *chip)
{
return container_of(chip, struct berlin_pwm_chip, chip);
}
static inline u32 berlin_pwm_readl(struct berlin_pwm_chip *chip,
unsigned int channel, unsigned long offset)
{
return readl_relaxed(chip->base + channel * 0x10 + offset);
}
static inline void berlin_pwm_writel(struct berlin_pwm_chip *chip,
unsigned int channel, u32 value,
unsigned long offset)
{
writel_relaxed(value, chip->base + channel * 0x10 + offset);
}
static int berlin_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
{
struct berlin_pwm_channel *channel;
channel = kzalloc(sizeof(*channel), GFP_KERNEL);
if (!channel)
return -ENOMEM;
return pwm_set_chip_data(pwm, channel);
}
static void berlin_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
{
struct berlin_pwm_channel *channel = pwm_get_chip_data(pwm);
pwm_set_chip_data(pwm, NULL);
kfree(channel);
}
static int berlin_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm_dev,
int duty_ns, int period_ns)
{
struct berlin_pwm_chip *pwm = to_berlin_pwm_chip(chip);
bool prescale_4096 = false;
u32 value, duty, period;
u64 cycles;
cycles = clk_get_rate(pwm->clk);
cycles *= period_ns;
do_div(cycles, NSEC_PER_SEC);
if (cycles > BERLIN_PWM_MAX_TCNT) {
prescale_4096 = true;
cycles >>= 12; // Prescaled by 4096
if (cycles > BERLIN_PWM_MAX_TCNT)
return -ERANGE;
}
period = cycles;
cycles *= duty_ns;
do_div(cycles, period_ns);
duty = cycles;
value = berlin_pwm_readl(pwm, pwm_dev->hwpwm, BERLIN_PWM_CONTROL);
if (prescale_4096)
value |= BERLIN_PWM_PRESCALE_4096;
else
value &= ~BERLIN_PWM_PRESCALE_4096;
berlin_pwm_writel(pwm, pwm_dev->hwpwm, value, BERLIN_PWM_CONTROL);
berlin_pwm_writel(pwm, pwm_dev->hwpwm, duty, BERLIN_PWM_DUTY);
berlin_pwm_writel(pwm, pwm_dev->hwpwm, period, BERLIN_PWM_TCNT);
return 0;
}
static int berlin_pwm_set_polarity(struct pwm_chip *chip,
struct pwm_device *pwm_dev,
enum pwm_polarity polarity)
{
struct berlin_pwm_chip *pwm = to_berlin_pwm_chip(chip);
u32 value;
value = berlin_pwm_readl(pwm, pwm_dev->hwpwm, BERLIN_PWM_CONTROL);
if (polarity == PWM_POLARITY_NORMAL)
value &= ~BERLIN_PWM_INVERT_POLARITY;
else
value |= BERLIN_PWM_INVERT_POLARITY;
berlin_pwm_writel(pwm, pwm_dev->hwpwm, value, BERLIN_PWM_CONTROL);
return 0;
}
static int berlin_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm_dev)
{
struct berlin_pwm_chip *pwm = to_berlin_pwm_chip(chip);
u32 value;
value = berlin_pwm_readl(pwm, pwm_dev->hwpwm, BERLIN_PWM_EN);
value |= BERLIN_PWM_ENABLE;
berlin_pwm_writel(pwm, pwm_dev->hwpwm, value, BERLIN_PWM_EN);
return 0;
}
static void berlin_pwm_disable(struct pwm_chip *chip,
struct pwm_device *pwm_dev)
{
struct berlin_pwm_chip *pwm = to_berlin_pwm_chip(chip);
u32 value;
value = berlin_pwm_readl(pwm, pwm_dev->hwpwm, BERLIN_PWM_EN);
value &= ~BERLIN_PWM_ENABLE;
berlin_pwm_writel(pwm, pwm_dev->hwpwm, value, BERLIN_PWM_EN);
}
static const struct pwm_ops berlin_pwm_ops = {
.request = berlin_pwm_request,
.free = berlin_pwm_free,
.config = berlin_pwm_config,
.set_polarity = berlin_pwm_set_polarity,
.enable = berlin_pwm_enable,
.disable = berlin_pwm_disable,
.owner = THIS_MODULE,
};
static const struct of_device_id berlin_pwm_match[] = {
{ .compatible = "marvell,berlin-pwm" },
{ },
};
MODULE_DEVICE_TABLE(of, berlin_pwm_match);
static int berlin_pwm_probe(struct platform_device *pdev)
{
struct berlin_pwm_chip *pwm;
struct resource *res;
int ret;
pwm = devm_kzalloc(&pdev->dev, sizeof(*pwm), GFP_KERNEL);
if (!pwm)
return -ENOMEM;
res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
pwm->base = devm_ioremap_resource(&pdev->dev, res);
if (IS_ERR(pwm->base))
return PTR_ERR(pwm->base);
pwm->clk = devm_clk_get(&pdev->dev, NULL);
if (IS_ERR(pwm->clk))
return PTR_ERR(pwm->clk);
ret = clk_prepare_enable(pwm->clk);
if (ret)
return ret;
pwm->chip.dev = &pdev->dev;
pwm->chip.ops = &berlin_pwm_ops;
pwm->chip.base = -1;
pwm->chip.npwm = 4;
pwm->chip.of_xlate = of_pwm_xlate_with_flags;
pwm->chip.of_pwm_n_cells = 3;
ret = pwmchip_add(&pwm->chip);
if (ret < 0) {
dev_err(&pdev->dev, "failed to add PWM chip: %d\n", ret);
clk_disable_unprepare(pwm->clk);
return ret;
}
platform_set_drvdata(pdev, pwm);
return 0;
}
static int berlin_pwm_remove(struct platform_device *pdev)
{
struct berlin_pwm_chip *pwm = platform_get_drvdata(pdev);
int ret;
ret = pwmchip_remove(&pwm->chip);
clk_disable_unprepare(pwm->clk);
return ret;
}
#ifdef CONFIG_PM_SLEEP
static int berlin_pwm_suspend(struct device *dev)
{
struct berlin_pwm_chip *pwm = dev_get_drvdata(dev);
unsigned int i;
for (i = 0; i < pwm->chip.npwm; i++) {
struct berlin_pwm_channel *channel;
channel = pwm_get_chip_data(&pwm->chip.pwms[i]);
if (!channel)
continue;
channel->enable = berlin_pwm_readl(pwm, i, BERLIN_PWM_ENABLE);
channel->ctrl = berlin_pwm_readl(pwm, i, BERLIN_PWM_CONTROL);
channel->duty = berlin_pwm_readl(pwm, i, BERLIN_PWM_DUTY);
channel->tcnt = berlin_pwm_readl(pwm, i, BERLIN_PWM_TCNT);
}
clk_disable_unprepare(pwm->clk);
return 0;
}
static int berlin_pwm_resume(struct device *dev)
{
struct berlin_pwm_chip *pwm = dev_get_drvdata(dev);
unsigned int i;
int ret;
ret = clk_prepare_enable(pwm->clk);
if (ret)
return ret;
for (i = 0; i < pwm->chip.npwm; i++) {
struct berlin_pwm_channel *channel;
channel = pwm_get_chip_data(&pwm->chip.pwms[i]);
if (!channel)
continue;
berlin_pwm_writel(pwm, i, channel->ctrl, BERLIN_PWM_CONTROL);
berlin_pwm_writel(pwm, i, channel->duty, BERLIN_PWM_DUTY);
berlin_pwm_writel(pwm, i, channel->tcnt, BERLIN_PWM_TCNT);
berlin_pwm_writel(pwm, i, channel->enable, BERLIN_PWM_ENABLE);
}
return 0;
}
#endif
static SIMPLE_DEV_PM_OPS(berlin_pwm_pm_ops, berlin_pwm_suspend,
berlin_pwm_resume);
static struct platform_driver berlin_pwm_driver = {
.probe = berlin_pwm_probe,
.remove = berlin_pwm_remove,
.driver = {
.name = "berlin-pwm",
.of_match_table = berlin_pwm_match,
.pm = &berlin_pwm_pm_ops,
},
};
module_platform_driver(berlin_pwm_driver);
MODULE_AUTHOR("Antoine Tenart <antoine.tenart@free-electrons.com>");
MODULE_DESCRIPTION("Marvell Berlin PWM driver");
MODULE_LICENSE("GPL v2");