Re: [PATCH v3 1/4] pwm: Add support for Meson PWM Controller

Neil Armstrong
2016-09-06T08:36:44Z

Hi Thierry,

On 09/05/2016 11:00 AM, Thierry Reding wrote:
> On Mon, Aug 22, 2016 at 05:36:30PM +0200, Neil Armstrong wrote:
>> Add support for the PWM controller found in the Amlogic SoCs.
>> This driver supports the Meson8b and GXBB SoCs.
>>
>> Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
>> ---
>> drivers/pwm/Kconfig | 9 +
>> drivers/pwm/Makefile | 1 +
>> drivers/pwm/pwm-meson.c | 528 ++++++++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 538 insertions(+)
>> create mode 100644 drivers/pwm/pwm-meson.c
>
> Hi Neil,
>
> sorry for taking so long to review this. I had actually started to write
> a review email since I had noticed a couple of slight oddities about the
> driver structure (primarily this was about how channel-specific data was
> split between struct meson_pwm_channel and struct meson_pwm_chip), but I
> ended up making some changes to the driver in order to see what my
> suggestions would look like, and if they would indeed improve things.
> But once I had done that, I thought it a bit pointless to make that into
> review comments and decided to just push what I had done and ask you to
> take a look, and if you had no objections to the changes take the driver
> for a spin to see if it still worked as expected.

We re-run our tests and I found 2 bugs, the first one is in meson_pwm_enable(),
only the channel A was setup, the fix is :

static void meson_pwm_enable(...)
- u32 value, clk_shift, clk_enable, enable;
+ u32 reg, value, clk_shift, clk_enable, enable;

switch (id) {
case 0:
[...]
+ reg = REG_PWM_A;
break;
case 1:
[...]
+ reg = REG_PWM_B;
break;
}
[...]
- writel(value, meson->base + REG_PWM_A);
+ writel(value, meson->base + reg);

The second bug is in probe(), I understand the point to allocate dynamically the channels
and attach them to each pwm chip, but when calling meson_pwm_init_channels() we get an OOPS
because meson->chip.pwms[i] are allocated in pwmchip_add().
Moving meson_pwm_init_channels() would fix this, but in case of a clk PROBE_DEFER, we would need
to remove back the pwmchip, which is a quite a bad design decision....

The smartest fix I found was to allocate channels in probe, init them them attach them after pwmchip_add():

static int meson_pwm_init_channels(..., struct meson_pwm_channel *channels)
{
+ struct meson_pwm_channel *channels;
[...]
- for (i = 0; i < meson->chip.npwm; i++) {
- struct pwm_device *pwm = &meson->chip.pwms[i];
- struct meson_pwm_channel *channel;
-
- channel = devm_kzalloc(dev, sizeof(*channel), GFP_KERNEL);
- if (!channel)
- return -ENOMEM;
+ if (!channels)
+ return -EINVAL;

+ for (i = 0; i < meson->chip.npwm; i++) {
[...]
+ memset(&channels[i], 0, sizeof(struct meson_pwm_channel));
[...]
//Rename "channel->" into "channels[i]."//
[...]
- pwm_set_chip_data(pwm, channel);
}

return 0;
}

+static void meson_pwm_add_channels_data(struct meson_pwm *meson,
+ struct meson_pwm_channel *channels)
+{
+ unsigned int i;
+
+ for (i = 0; i < meson->chip.npwm; i++)
+ pwm_set_chip_data(&meson->chip.pwms[i], &channels[i]);
+}

static int meson_pwm_probe(struct platform_device *pdev)
{
+ struct meson_pwm_channel *channels;
[...]
- err = meson_pwm_init_channels(meson);
- if (err < 0)
- return err;
-
meson->chip.dev = &pdev->dev;
[...]
meson->chip.of_pwm_n_cells = 3;

+ channels = devm_kmalloc_array(&pdev->dev, 2, sizeof(*meson),
+ GFP_KERNEL);
+ if (!channels)
+ return -ENOMEM;
+
+ err = meson_pwm_init_channels(meson, channels);
+ if (err < 0)
+ return err;
+
err = pwmchip_add(&meson->chip);
[...]
+ meson_pwm_add_channels_data(meson, channels);
+
platform_set_drvdata(pdev, meson);

return 0;
}

The fix driver is in a separate branch, rebased on your for-next :
https://github.com/superna9999/linux/tree/amlogic/v4.8/pwm-for-next

and in a signed tag I can transform in a pull request if needed :
https://github.com/superna9999/linux/releases/tag/amlogic/v4.8/pwm-for-next-for-v4

[...]
>
> I've pushed my modifications to the driver to the linux-pwm repository:
>
> https://git.kernel.org/cgit/linux/kernel/git/thierry.reding/linux-pwm.git/log/?h=for-next
>
> Alternatively you can also take a look at the for-4.9/drivers branch,
> but they're currently the same thing.
>
> Thierry
>

Thanks,
Neil