Re: [RFC PATCH 2/7] include/linux: add headers for drivers/zio

Federico Vaga
2011-11-28T15:05:26Z

2011/11/27 Greg KH <greg@kroah.com>:
> On Sat, Nov 26, 2011 at 10:46:48PM +0100, Federico Vaga wrote:
>> In data sabato 26 novembre 2011 12:02:16, Greg KH ha scritto:
>> > On Sat, Nov 26, 2011 at 06:30:31PM +0100, Alessandro Rubini wrote:
>> > > +/*
>> > > + * We use the same functions to deal with attributes, but the structures
>> > > + * we act on may be different (dev, cset, channel). Thus, all structures
>> > > + * begin with the type identifier, and zio_obj_head is used in
>> > > container_of + */
>> >
>> > Because you are using container_of, you don't have to have the structure
>> > at the beginning of the structure it is included in, right?
>>
>> Different structures have similar features and we use zio_obj_head->zobj_type to
>> identify the correct container_of to apply.  Sometimes we use the head only, so
>> we delay container_of later.
>
> That's usually not a good idea, which is why we don't do it generally in
> the rest of the kernel.
>
> You should "almost" always already know the type of device you are
> pointing to when pointing to it, so that you can properly dereference
> it.  I think you are doing the right thing here, but the true test would
> be to move that structure somewhere else other than "first" and see what
> breaks.

One of our next commits will be the moving of this field somewhere
else in the structures for bug finding. Thank you for the suggestion.

>> > > +enum zio_object_type {
>> > > +   ZNONE = 0,      /* reserved for non zio object */
>> > > +   ZDEV, ZCSET, ZCHAN,
>> > > +   ZTRIG, ZTI,     /* trigger and trigger instance */
>> > > +   ZBUF, ZBI,      /* buffer and buffer instance */
>> > > +};
>> > > +
>> > > +/* zio_obj_head is for internal use only, as explained above */
>> > > +struct zio_obj_head {
>> > > +   struct kobject          kobj;
>> > > +   enum zio_object_type    zobj_type;
>> > > +   char                    name[ZIO_NAME_LEN];
>> > > +};
>> > > +#define to_zio_head(_kobj) container_of(_kobj, struct zio_obj_head, kobj)
>> > > +#define to_zio_dev(_kobj) container_of(_kobj, struct zio_device,
>> > > head.kobj) +#define to_zio_cset(_kobj) container_of(_kobj, struct
>> > > zio_cset, head.kobj) +#define to_zio_chan(_kobj) container_of(_kobj,
>> > > struct zio_channel, head.kobj)
>> > Why are you using a "raw" kobject and not 'struct device' instead?
>>
>> The device way was experimented and we can move in that direction. I also
>> tried a mixed solution with device and kobject, because not all the zio objects
>> can be device.
>>
>> I decided to use the kobject way because it was an easier and flexible solution
>> for a fast development.
>>
>> > If you use a kobject, you loose all of the device tree information that a
>> > real struct device provides to userspace,
>>
>> You mean the device sysfs tree? Acctually we don't need that information
>
> Yes you do, you are already using it, right?
>
> By using kobjects you "skip" notifying userspace the whole tree and it
> only knows about "parts" of it, which is why you should not do this.
>
>> > and can only cause confusion in the long run.
>>
>> I think it can be confusing to declare a device what is not a device, for
>> example: buffer, trigger, channel-set (maybe in some
>> sense can be a device) and channel
>
> Nope, they seem like "devices" to me in that you want them showing up in
> sysfs, which is why you used kobjects, right?  Because of that, you are
> in the device tree, so you need to use a 'struct device'.
>
>> > This also will provide you the "type" and name that you are needing
>> > here, as well as lots of other good things (properly formatted logging
>> > messages, uevents, etc.)
>>
>> If you refer to device_type, I think it is too complex for our purpose (also
>> tried during the device "experiment"), we only need to recognize a zio object,
>> we don't need al the stuff within device_type.
>>
>> You are right, device is full of great things and the migration to device is
>> always a point of discussion, but actually kobject meet well with our needs.
>
> I beg to differ.
>
>> > Please consider moving to that instead.
>>
>> We can re-evaluate and better explain the choice if kobj is still the
>> preferrable one
>
> kobject is not the preferrable one, sorry.

I don't reply to each point because I suppose the point is: "kobject
or device for sysfs?", maybe I misunderstood.
I know that it is not a good thing reply with a new question, but I
think it can clarify the problem.

Why device is it preferred to export some sysfs attributes when sysfs
ask me kobject?

As I tested both device and kobject, I'll do a parallel implementation
with device.

> greg k-h
>

--
Federico Vaga