Re: [PATCH v2] mtd: implement proper partition handling

[Boris Brezillon <boris.brezillon@collabora.com>]

On Fri, 10 Jan 2020 09:28:55 +0100
Miquel Raynal <miquel.raynal@bootlin.com> wrote:

> Hi Boris,
> 
> Boris Brezillon <boris.brezillon@collabora.com> wrote on Thu, 9 Jan
> 2020 20:37:22 +0100:
> 
> > On Thu, 9 Jan 2020 20:23:58 +0100
> > Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> >   
> > > Hi Boris,
> > > 
> > > Boris Brezillon <boris.brezillon@collabora.com> wrote on Thu, 9 Jan
> > > 2020 20:13:55 +0100:
> > >     
> > > > On Thu, 9 Jan 2020 19:45:56 +0100
> > > > Miquel Raynal <miquel.raynal@bootlin.com> wrote:
> > > >       
> > > > > Hi Richard,
> > > > > 
> > > > > Richard Weinberger <richard@nod.at> wrote on Thu, 9 Jan 2020 19:43:04
> > > > > +0100 (CET):
> > > > >         
> > > > > > Miquel,
> > > > > > 
> > > > > > ----- Ursprüngliche Mail -----          
> > > > > > >> What problem does this solve?
> > > > > > >> ...beside of a nice diffstat which removes more than it adds. :-)            
> > > > > > > 
> > > > > > > It is much easier to escalade to the top most "master" device when
> > > > > > > there are multiple levels of partitioning, which was not cleanly
> > > > > > > described IMHO. Also it is already used in the MLC-in-pseudo-SLC-mode
> > > > > > > series :)            
> > > > > > 
> > > > > > Ok. In fact I "found" this patch my looking at the SLC emulation patches.
> > > > > >           
> > > > > > >> > +static inline struct mtd_info *mtd_get_master(struct mtd_info *mtd)
> > > > > > >> > +{
> > > > > > >> > +	while (mtd->parent)
> > > > > > >> > +		mtd = mtd->parent;
> > > > > > >> > +
> > > > > > >> > +	return mtd;
> > > > > > >> > +}            
> > > > > > >> 
> > > > > > >> So, parent == master?            
> > > > > > > 
> > > > > > > top most parent (the one without parent) == master !
> > > > > > >             
> > > > > > >> 
> > > > > > >> When I create a MTD ontop of UBI using gluebi, who will be parent/master?            
> > > > > > > 
> > > > > > > I don't really understand the issue here?            
> > > > > > 
> > > > > > Let's say I have mtd0 with an ubi and a volume "xxx". After enabling
> > > > > > gluebi a new mtd1 will arrive on the system.
> > > > > > The stacking is mtd0 -> ubi (volume xxx) -> mtd1.          
> > > > > 
> > > > > This is much clearer, thanks!
> > > > >         
> > > > > > Is now a relationship between mtd1 and mtd0?          
> > > > > 
> > > > > No there is none. 
> > > > >         
> > > > > > I'd expect mtd1's parent being mtd0.          
> > > > > 
> > > > > This would be a new feature, right? I don't think it is the case today.        
> > > > 
> > > > We definitely don't want mtd1 to appear as a partition of mtd0 in that
> > > > case (blocks in mtd1 can't be mapped to blocks in mtd0 without the UBI
> > > > layer being involved). Maybe it'd be clearer if we move the parent
> > > > field to mtd_part and add an MTD_IS_PARTITION flag. Or maybe we can
> > > > just choose a better name.      
> > > 
> > > I prefer the name change. I think the current struct organization
> > > is right. What do you suggest?    
> > 
> > I don't have a better name, sorry.  
> 
> Actually I find ->parent totally descriptive, and in Richard's example,
> I would not call mtd1 as mtd0's parent, it's more like a "top virtual
> device" but certainly not a "direct" parent.

It depends what kind of parenting we're talking about :P. From the
device model perspective, mtd1's parent is ubi0. From the MTD partition
perspective, mtd1 has no parent (it's a master MTD device). The problem
here is that 'parent' is vague enough that it lets people think it's
representing more than a partition -> partition_parent relationship,
hence my suggestion to move the field to mtd_part so it can be
namespaced in mtd->part.parent.

> 
> mtd->direct_parent would work but I think it is a bit too long and most
> of the people would not understand why we call it this way, instead of
> just "parent".

->partition_parent would be more accurate, but I agree with you, I'm
not a big fan of those long names. Maybe ->container?

> 
> I would like to take this patch into 5.6, so please tell me what you
> prefer ("parent" being the most straightforward and simple solution to
> me :) )

I think the most important thing here is documenting what this field
is representing so people can refer to the doc if they get confused.

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