[PATCH] mtd: Add sysfs attribute for mtd OOB available size

[xiaolei li]

On Tue, 2018-04-03 at 10:43 +0200, Boris Brezillon wrote:
> On Tue, 3 Apr 2018 10:46:58 +0800
> xiaolei li <xiaolei.li at mediatek.com> wrote:
> 
> > Hello Boris,
> > 
> > On Mon, 2018-04-02 at 14:15 +0200, Boris Brezillon wrote:
> > > Hi Xiaolei,
> > > 
> > > On Mon, 2 Apr 2018 16:20:10 +0800
> > > Xiaolei Li <xiaolei.li at mediatek.com> wrote:
> > >   
> > > > Expose mtd OOB available size by sysfs file. Then users can get available
> > > > OOB size by accessing /sys/class/mtd/mtdX/oobavail.  
> > > 
> > > May I ask why you need to expose that? I'm not against exposing new
> > > things through sysfs, but since this is part of the ABI, I'd like to be
> > > sure we actually need it.
> > >   
> > That is user-space can write OOB data through ioctl MEMWRITE now.
> > If OOB operation mode is MTD_OPS_AUTO_OOB, user should know how many
> > OOB available bytes it can use. But I didn't find a way to get it. (If
> > there is already a method, please kindly let me know, thanks.)
> 
> Nope. We have the ECCGETLAYOUT ioctl, but it's not reliable because of
> the MTD_MAX_OOBFREE_ENTRIES limitation.
> 
> > 
> > One problem I know is to do Jffs2 type flash erase using MTD user-space
> > tool flash_erase. flash_erase tool will program "cleanmarker" into OOB
> > free area, but it can not get OOB available size.
> 
> That's a good reason to expose this property indeed. Do you plan to fix
> mtd-utils to use the sysfs value when it's available? Also, do we need
> to expose the ECC/free layout as done with the
> ECCGETLAYOUT/MEMGETOOBSEL ioctls?
> 
Yes. I plan to fix mtd-utils problem if this change is accepted.

struct nand_oobinfo and struct nand_ecclayout_user in mtd-abi.h are
obsoleted, and user can not get oob layout by ioctl method now.
sysfs seems a method to fetch oob layout. But I don't know which
user-space case will use oob layout now. Maybe expose them in the future
if they are really needed?

Thanks,
Xiaolei