Re: [PATCH v2] pwm: Create device class for pwm channels

From: Thierry Reding
Date: Mon Sep 05 2016 - 11:15:19 EST

On Mon, Sep 05, 2016 at 04:41:39PM +0200, Thierry Reding wrote:
> On Mon, Jul 18, 2016 at 03:14:50PM -0700, David Hsu wrote:
[...]
> > diff --git a/drivers/pwm/sysfs.c b/drivers/pwm/sysfs.c
[...]
> > @@ -248,9 +254,11 @@ static int pwm_export_child(struct device *parent, struct pwm_device *pwm)
> >
> > export->child.release = pwm_export_release;
> > export->child.parent = parent;
> > + export->child.type = &pwm_channel_type;
> > export->child.devt = MKDEV(0, 0);
> > + export->child.class = &pwm_class;
>
> This particular change isn't going to work, unfortunately. Children of a
> PWM chip, i.e. PWM devices (or channels) are not the same as chips. The
> above, however, will cause the attributes associated with a PWM chip to
> be associated with each PWM device as well. For example it will cause a
> PWM device to expose an "export" attribute in userspace. Writing to that
> file will give you a nice oops, along these lines:
>
> [ 89.631855] Unable to handle kernel NULL pointer dereference at virtual address 00000014
> [ 89.640146] pgd = c2b16700
> [ 89.642879] [00000014] *pgd=82b74003, *pmd=f4dd8003
> [ 89.647830] Internal error: Oops: 207 [#1] PREEMPT SMP ARM
> [ 89.653330] Modules linked in: nouveau tegra_drm ttm drm_kms_helper cfbfillrect syscopyarea cfbimgblt sysfillrect sysimgblt fb_sys_fops cfbcopyarea drm
> [ 89.667194] CPU: 3 PID: 284 Comm: sh Not tainted 4.8.0-rc3-next-20160825-00026-g7065511b7003-dirty #82
> [ 89.676507] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
> [ 89.682787] task: c2b39500 task.stack: c3034000
> [ 89.687327] PC is at export_store+0x34/0x170
> [ 89.691598] LR is at _kstrtoull+0x2c/0x70
> [ 89.695607] pc : [<c04b1cb0>] lr : [<c048a0e4>] psr: 60000013
> [ 89.695607] sp : c3035ea0 ip : c04b1c7c fp : bec8eb0c
> [ 89.707068] r10: ee1e9b8c r9 : c3035f80 r8 : 00000002
> [ 89.712287] r7 : c2b70800 r6 : 00000000 r5 : ee1e9b80 r4 : 00000000
> [ 89.718806] r3 : 00000000 r2 : 00000000 r1 : 00000000 r0 : 00000000
> [ 89.725327] Flags: nZCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
> [ 89.732453] Control: 30c5387d Table: 82b16700 DAC: 55555555
> [ 89.738193] Process sh (pid: 284, stack limit = 0xc3034210)
> [ 89.743758] Stack: (0xc3035ea0 to 0xc3036000)
> [ 89.748116] 5ea0: ee1e9b8c 00000000 0014e408 00000002 ee1e9b80 00000000 00000000 ee19f180
> [ 89.756286] 5ec0: c3035f80 c035fc98 00000000 00000000 c3070180 c035fbd8 0014e408 c3035f80
> [ 89.764456] 5ee0: 00000000 00000002 00000000 c030167c 032cf000 c109bf44 c109bf44 c02f88b0
> [ 89.772625] 5f00: ee8270c0 c02fac24 00000001 c3035f10 ef05c9e0 c0300fd8 c0327c08 c2b7b000
> [ 89.780794] 5f20: 0000000a c2b7e000 c3184ec0 0000000a 00000400 c2b7e040 00000000 c3070180
> [ 89.788964] 5f40: 00000002 0014e408 c3035f80 00000000 00000002 c0302464 00000001 0000000a
> [ 89.797132] 5f60: c2d076c0 c3070180 c3070180 00000000 00000000 0014e408 00000002 c03031b0
> [ 89.805302] 5f80: 00000000 00000000 00000014 00000002 0014e408 b6e74d50 00000004 c02075e4
> [ 89.813470] 5fa0: c3034000 c0207440 00000002 0014e408 00000001 0014e408 00000002 00000000
> [ 89.821638] 5fc0: 00000002 0014e408 b6e74d50 00000004 00000002 00000004 b6f0e000 bec8eb0c
> [ 89.829808] 5fe0: 00000000 bec8ea64 b6d9ffa4 b6df970c 60000010 00000001 0b0a0908 0f0e0d0c
> [ 89.837996] [<c04b1cb0>] (export_store) from [<c035fc98>] (kernfs_fop_write+0xc0/0x1cc)
> [ 89.846005] [<c035fc98>] (kernfs_fop_write) from [<c030167c>] (__vfs_write+0x1c/0x114)
> [ 89.853940] [<c030167c>] (__vfs_write) from [<c0302464>] (vfs_write+0xa4/0x168)
> [ 89.861252] [<c0302464>] (vfs_write) from [<c03031b0>] (SyS_write+0x3c/0x90)
> [ 89.868304] [<c03031b0>] (SyS_write) from [<c0207440>] (ret_fast_syscall+0x0/0x34)
> [ 89.875883] Code: ebff6164 e2506000 ba000039 e59d1004 (e5943014)
> [ 89.882225] ---[ end trace 716eda7e65a4136a ]---
>
> Given that we need a class associated with a device in order for it to
> generate uevents (why is that so, by the way?), I think we'd need to add
> a separate class implementation for PWM devices. That has the downside
> of adding yet another subdirectory to /sys/class, but I can't really
> think of another way of achieving what you need here.
>
> Greg, any ideas?

*sigh*, nevermind. I realized too late that I hadn't fully applied your
patch and the change to use device_create_with_groups() actually gets
rid of this.

Unfortunately....

> > index 01695d4..cb2b376 100644
> > --- a/drivers/pwm/sysfs.c
> > +++ b/drivers/pwm/sysfs.c
> > @@ -23,6 +23,8 @@
> > #include <linux/kdev_t.h>
> > #include <linux/pwm.h>
> >
> > +static struct class pwm_class;
> > +
> > struct pwm_export {
> > struct device child;
> > struct pwm_device *pwm;
> > @@ -222,6 +224,10 @@ static struct attribute *pwm_attrs[] = {
> > };
> > ATTRIBUTE_GROUPS(pwm);
> >
> > +static const struct device_type pwm_channel_type = {
> > + .name = "pwm_channel",
> > +};
>
> In order to do some tracing, I ended up implementing the ->uevent()
> callback of struct device_type. What I noticed when exporting a PWM is
> that that ->uevent() gets called recursively and the operation never
> finishes. I have no idea why that happens, though.

... that's still there. However the output is now somewhat cleaner and
the ->uevent() callback doesn't seem to be called recursively but rather
repeatedly (until I unexport the PWM again).

Let me investigate a little more where this is coming from. For
reference, I have the below on top of your patch.

Thierry

--- >8 ---
diff --git a/drivers/pwm/sysfs.c b/drivers/pwm/sysfs.c
index 2ab5ed61d496..45e56a04f091 100644
--- a/drivers/pwm/sysfs.c
+++ b/drivers/pwm/sysfs.c
@@ -241,8 +241,17 @@ static struct attribute *pwm_attrs[] = {
};
ATTRIBUTE_GROUPS(pwm);

+static int pwm_uevent(struct device *dev, struct kobj_uevent_env *env)
+{
+ int err = 0;
+ dev_info(dev, "> %s(dev=%p, env=%p)\n", __func__, dev, env);
+ dev_info(dev, "< %s() = %d\n", __func__, err);
+ return err;
+}
+
static const struct device_type pwm_channel_type = {
- .name = "pwm_channel",
+ .name = "pwm_channel",
+ .uevent = pwm_uevent,
};

static void pwm_export_release(struct device *child)

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