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

[Greg KH]

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@xxxxxxxxx> wrote:
> > 
> > On Tue, Apr 7, 2015 at 2:23 PM, Pantelis Antoniou
> > <pantelis.antoniou@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> >> ---
> >> 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.
> > 
> 
> Itâs the same reason that the device tree core is using it.

It is doing that for "raw" device tree files, not individual attributes,
right?

> 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 thatâs 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 thatâs going to be more complicated not less.

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.

thanks,

greg k-h
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