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

[Miquel Raynal <miquel.raynal@bootlin.com>]

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.

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".

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 :) )

Thanks,
Miquèl

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