Re: [PATCH v2 3/4] of: overlay: Add sysfs attributes

Pantelis Antoniou
2015-04-23T12:39:32Z

Hi Greg,

> On Apr 23, 2015, at 15:33 , Greg KH <greg@kroah.com> wrote:
>
> On Thu, Apr 23, 2015 at 03:00:03PM +0300, Pantelis Antoniou wrote:
>> Hi Rob,
>>
>>> On Apr 15, 2015, at 04:27 , Rob Herring <robherring2@gmail.com> wrote:
>>>
>>> On Tue, Apr 7, 2015 at 2:23 PM, Pantelis Antoniou
>>> <pantelis.antoniou@konsulko.com> wrote:
>>>> Implement a number of sysfs attributes for overlays.
>>>>
>>>> * A throw once master enable switch to protect against any
>>>> further overlay applications if the administrator desires so.
>>>
>>> This one should be a separate patch.
>>>
>>
>> OK.
>>
>>>> * A per overlay targets sysfs attribute listing the targets of
>>>> the installed overlay.
>>>
>>> What are targets? "targets lists targets" does not help me. The
>>> documentation doesn't help me either.
>>>
>>
>> It lists the targets of the overlay that has been applied. What do
>> you need in order to be helped? I mean what do you want listed?
>>
>>>> * A per overlay can_remove sysfs attribute that reports whether
>>>> the overlay can be removed or not due to another overlapping overlay.
>>>>
>>>> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
>>>> ---
>>>> drivers/of/overlay.c | 167 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>> 1 file changed, 166 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
>>>> index f17f5ef..c54d097 100644
>>>> --- a/drivers/of/overlay.c
>>>> +++ b/drivers/of/overlay.c
>>>> @@ -21,6 +21,7 @@
>>>> #include <linux/err.h>
>>>> #include <linux/idr.h>
>>>> #include <linux/sysfs.h>
>>>> +#include <linux/atomic.h>
>>>>
>>>> #include "of_private.h"
>>>>
>>>> @@ -55,8 +56,12 @@ struct of_overlay {
>>>> struct kobject kobj;
>>>> };
>>>>
>>>> +/* master enable switch; once set to 0 can't be re-enabled */
>>>> +static atomic_t ov_enable = ATOMIC_INIT(1);
>>>> +
>>>> static int of_overlay_apply_one(struct of_overlay *ov,
>>>> struct device_node *target, const struct device_node *overlay);
>>>> +static int overlay_removal_is_ok(struct of_overlay *ov);
>>>>
>>>> static int of_overlay_apply_single_property(struct of_overlay *ov,
>>>> struct device_node *target, struct property *prop)
>>>> @@ -345,6 +350,144 @@ static struct kobj_type of_overlay_ktype = {
>>>>
>>>> static struct kset *ov_kset;
>>>>
>>>> +static ssize_t enable_read(struct file *filp, struct kobject *kobj,
>>>> + struct bin_attribute *bin_attr, char *buf,
>>>> + loff_t offset, size_t count)
>>>> +{
>>>> + char tbuf[3];
>>>> +
>>>> + if (offset < 0)
>>>> + return -EINVAL;
>>>> +
>>>> + if (offset >= sizeof(tbuf))
>>>> + return 0;
>>>> +
>>>> + if (count > sizeof(tbuf) - offset)
>>>> + count = sizeof(tbuf) - offset;
>>>> +
>>>> + /* fill in temp */
>>>> + tbuf[0] = '0' + atomic_read(&ov_enable);
>>>> + tbuf[1] = '\n';
>>>> + tbuf[2] = '\0';
>>>> +
>>>> + /* copy to buffer */
>>>> + memcpy(buf, tbuf + offset, count);
>>>> +
>>>> + return count;
>>>> +}
>>>> +
>>>> +static ssize_t enable_write(struct file *filp, struct kobject *kobj,
>>>> + struct bin_attribute *bin_attr, char *buf,
>>>> + loff_t off, size_t count)
>>>> +{
>>>> + unsigned int new_enable;
>>>> +
>>>> + if (off != 0 || (buf[0] != '0' && buf[0] != '1'))
>>>> + return -EINVAL;
>>>> +
>>>> + new_enable = (unsigned int)(buf[0] - '0');
>>>> + if (new_enable > 1)
>>>> + return -EINVAL;
>>>> +
>>>> + /* NOP for same value */
>>>> + if (new_enable == atomic_read(&ov_enable))
>>>> + return count;
>>>> +
>>>> + /* if we've disabled it, no going back */
>>>> + if (atomic_read(&ov_enable) == 0)
>>>> + return -EPERM;
>>>> +
>>>> + atomic_set(&ov_enable, new_enable);
>>>> + return count;
>>>> +}
>>>> +
>>>> +/* just a single char + '\n' + '\0' */
>>>> +static BIN_ATTR_RW(enable, 3);
>>>
>>> Why are you using bin attribute? You are complicating the
>>> implementation needlessly.
>>>
>>
>> It’s the same reason that the device tree core is using it.
>
> It is doing that for "raw" device tree files, not individual attributes,
> right?
>

Each property of a device tree is a binary attribute.

>> Believe it or not, this is the simplest way to do it.
>> If you take a look at the sysfs attribute implementation, the binary
>> implementation is the one that’s using the least amount of code.
>
> Then something is really wrong here.
>
>> To use a non-binary method we have to register per ktype sysfs_ops
>> and duplicate the way the non-binary attribute works.
>
> really? Again, something must be wrong.
>
>> For the gory details look at sysfs_add_file_mode_ns() in fs/sysfs/file.c
>>
>> I can add the sysfs_ops but that’s going to be more complicated not less.
>

Please take a look in linux/sysfs.h.
The non-binary sysfs accessors are all using some kind of other kobj;
for instance DEVICE_ATTR is using a device_attribute, etc.

For the overlay case, I’d have to create a of_overlay_attribute and work from
there.

> Only use binary sysfs files if you are accepting binary data directly
> from userspace and using it as a "pass-through" to the kernel.
>
> Otherwise just use a "normal" sysfs file. I don't understand the
> problem here, sysfs shouldn't be hard to use for simple attributes, that
> was not the goal here at all.
>

There is no generic (i.e. not kobj type specific), non-binary sysfs file interface right now.
I can add one for my case, but that’s more code.

It’s your call.

> thanks,
>
> greg k-h

Regards

— Pantelis