On Tue, Sep 06, 2016 at 11:14:45AM +0200, Neil Armstrong wrote:

On 09/06/2016 11:07 AM, Thierry Reding wrote:

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On Tue, Sep 06, 2016 at 10:36:49AM +0200, Neil Armstrong wrote:

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Hi Thierry,

[...]

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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....

Ah yes... that one again. I remember running into that a while ago with
some other driver. To be honest, I think that's a short-coming of the
PWM subsystem and the fix would be for PWM chip registration to be split
into two parts: pwm_chip_init() and pwm_chip_add(). That way, a chip
would be initialized using pwm_chip_init() where the pwms array would be
allocated, and pwm_chip_add() would register the chip with the system.

Currently a few drivers might be vulnerable to a race condition between
registration and implementation (i.e. PWM channels aren't fully set up
when they are exposed to users and sysfs).

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

[...]

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That's the race I was talking about above. I suppose it's not too big an
issue since other drivers seem to manage, so I'm going to merge your
fixed driver.

ok thanks !

I've made a few tiny changes (reg -> offset, temporary variable to track
&channels[i], ...) and pushed it all out. Hopefully that now fixes any
of the remaining issues.

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Unless you feel like taking a stab at the pwm_chip_init()/pwm_chip_add()
split, in which case your driver would be the first to be race-free. =)

Having he driver upstream is a priority, but having it completely
race-free would be great! I'll be happy to collaborate to a race-free
pwmchip probe somehow !

Fair enough. I'll do some prototyping and keep you in the loop if I come
up with something that I think will do.

Thierry
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