Re: [PATCH v3 2/3] mtd: spi-nor: add 4bit block protection support

[Jungseung Lee <js07.lee@xxxxxxxxxxx>]

Hi, Tudor and all

2020-02-10 (Mon), 11:26 +0000, Tudor.Ambarus@xxxxxxxxxxxxx:
> On Monday, February 10, 2020 12:29:34 PM EET Michael Walle wrote:
> > > It's bytes. Take a look at W25Q128JV. The sector size for this
> > > flash is
> > > 64KByte. The flash has 256 sectors. For this specific case:
> > > bp_slots_available = 6;
> > > bp_slots_needed = 8;
> > > 
> > > The if condition is true, so
> > > bp_slot_count = 6;
> > > bp_min_slot_size = 64k << (8 - 6); //256k
> > 
> > But nor->info->n_sectors is not 64k, its 256. Do you mean
> > sector_size
> > (like in
> > my example below? Then we are on the same page 
> 
> Indeed, there is a typo in the pseudo code; I'm happy that the
> example is 
> correct at least. I meant sector_size, not sectors. Now we should
> exercise the 
> logic to all the known (corner) cases. Maybe Jungseung will tell us
> if he 
> spots a flaw in the overall logic.
> 

I didn't find any flaw in this logic. But IMHO for the pseudo code.

bp_slots_available = (bp_mask >> shift) + 1 - 2;
bp_slots_needed = ilog2(nor->info->n_sectors);
 
if (bp_slots_needed > bp_slots_available) {  
    bp_slot_count = bp_slots_available;
    bp_min_slot_size = nor->info->sector_size <<
        (bp_slots_needed - bp_slots_available);
} else {
    bp_slot_count = bp_slots_needed;
    bp_min_slot_size = mtd->size >> bp_slot_count;
}

Probably we can use directly nor->info->sector_size for bp_min_slot_
size.

sector_size x n_sectors = mtd->size
mtd->size / n_sectors = mtd->size >>
ilog2(n_sectors) = sector_size

bp_slot_count is equal to log2(n_sectors) now so if we can trust the
value, we can also trust sector_size.

After change it,

if (bp_slots_needed > bp_slots_available)
    bp_min_slot_size = nor->info->sector_size <<
        (bp_slots_needed - bp_slots_available);
else
    bp_min_slot_size = nor->info->sector_size;

This is a comment from my previous mail.
> > The exact fact is that locks operate in two different ways
> > according to flash model.
> > 
> > (1) the smallest protected portion is fixed.
> >     for BP0-2 : 1/64
> >     for BP0-1 : 1/4
> > (2) the smallest protected portion is inversely propotional with
> > number of sectors.

(1) - if the slot is insufficient.
(2) - if the slot is sufficient.

>From the fact, that could be rewritten like below. I think it's more
intuitive one.

if (bp_slots_needed > bp_slots_available)    // (1)
    bp_min_slot_size = mtd->size >> bp_slots_available;
else                                         // (2)
    bp_min_slot_size = nor->info->sector_size;

We could also find a few flashes that does not following the overall
logic. For example, "en25qh256" and "en25qh16" which was manufactured
by EON. They are always following way (2) no matter what the number of
slot is. It seems that it could be handled like below with custom hook
later.

if (bp_slots_needed < bp_slots_available || force)
    bp_min_slot_size = nor->info->sector_size;
else
    bp_min_slot_size = mtd->size >> bp_slots_available;

> Cheers,
> ta
> 

Thanks,



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