linux-kernel - Re: [PATCH 3/4] mtd: mchp23k256: add partitioning support

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Message-ID: <20170601224712.6a81a458@bbrezillon>
Date:   Thu, 1 Jun 2017 22:47:12 +0200
From:   Boris Brezillon <boris.brezillon@...e-electrons.com>
To:     Brian Norris <computersforpeace@...il.com>
Cc:     Chris Packham <chris.packham@...iedtelesis.co.nz>,
        dwmw2@...radead.org, andrew@...n.ch, linux-mtd@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
Subject: Re: [PATCH 3/4] mtd: mchp23k256: add partitioning support

Le Thu, 1 Jun 2017 11:43:40 -0700,
Brian Norris <computersforpeace@...il.com> a écrit :

> On Wed, May 17, 2017 at 05:29:11PM +0200, Boris Brezillon wrote:
> > Hi Chris,
> > 
> > On Wed, 17 May 2017 17:39:07 +1200
> > Chris Packham <chris.packham@...iedtelesis.co.nz> wrote:
> >   
> > > Setting the of_node for the mtd device allows the generic mtd code to
> > > setup the partitions. Additionally we must specify a non-zero erasesize
> > > for the partitions to be writeable.
> > > 
> > > Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
> > > ---
> > >  drivers/mtd/devices/mchp23k256.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/drivers/mtd/devices/mchp23k256.c b/drivers/mtd/devices/mchp23k256.c
> > > index 2542f5b8b63f..02c6b9dcbd3e 100644
> > > --- a/drivers/mtd/devices/mchp23k256.c
> > > +++ b/drivers/mtd/devices/mchp23k256.c
> > > @@ -143,6 +143,7 @@ static int mchp23k256_probe(struct spi_device *spi)
> > >  
> > >  	data = dev_get_platdata(&spi->dev);
> > >  
> > > +	mtd_set_of_node(&flash->mtd, spi->dev.of_node);
> > >  	flash->mtd.dev.parent	= &spi->dev;
> > >  	flash->mtd.type		= MTD_RAM;
> > >  	flash->mtd.flags	= MTD_CAP_RAM;
> > > @@ -151,6 +152,10 @@ static int mchp23k256_probe(struct spi_device *spi)
> > >  	flash->mtd._read	= mchp23k256_read;
> > >  	flash->mtd._write	= mchp23k256_write;
> > >  
> > > +	flash->mtd.erasesize = PAGE_SIZE;
> > > +	while (flash->mtd.size & (flash->mtd.erasesize - 1))
> > > +		flash->mtd.erasesize >>= 1;
> > > +  
> > 
> > Can we fix allocate_partition() to properly handle the
> > master->erasesize == 0 case instead of doing that?  
> 
> Is everything actually ready for the eraseblock size to be 0? That would
> seem surprising to many applications, I would think. Can you, for
> instance, even use UBI on such a device?

Well, I think it's already broken. AFAICT this driver does not
implement ->_erase(), and mtd_erase() does not check if MTD_NO_ERASE is
set before calling mtd->_erase(), neither UBI does before calling
mtd_erase().

Between a NULL pointer exception and a div-by-zero exception, I can't
decide what is better :-).

IMO, we'd better add a check in UBI to refuse to attach a device with
MTD_NO_ERASE or mtd->erasesize == 0, and fix other places that don't
check erasesize value instead of putting a fake erasesize and using a
dummy ->_erase() implementation for those devices that simply can't be
erased.

We should also probably complain with -ENOTSUPP when someone calls
mtd_erase() on a device with MTD_NO_ERASE and add more checks in the
add_mtd_device() to detect drivers that don't have MTD_NO_ERASE set
and do not implement ->_erase() or leave ->erasesize to 0. 

> 
> BTW, I feel like this check is a little more natural to do with
> 'mtd->flags & MTD_NO_ERASE', rather than checking the (apparently
> meaningless) erasesize.

Fair enough.