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

[<Tudor.Ambarus@xxxxxxxxxxxxx>]

Hi, Michael,

On Monday, February 10, 2020 10:33:41 AM EET Michael Walle wrote:

cut

> > On Monday, February 3, 2020 3:56:58 PM EET Vignesh Raghavendra wrote:
> >> >>>>>>>>> /*
> >> >>>>>>>>> * Need smallest pow such that:
> >> >>>>>>>>> *
> >> >>>>>>>>> @@ -1908,7 +1972,17 @@ static int stm_lock(struct
> >> >>>>>>>>> spi_nor
> >> >>>>>>>>> *nor,
> >> >>>>>>>>> loff_t ofs, uint64_t len)
> >> >>>>>>>>> *   pow = ceil(log2(size / len)) = log2(size)
> >> >>>>>>>>> -
> >> >>>>>>>>> floor(log2(len))
> >> >>>>>>>>> */
> >> >>>>>>>>> pow = ilog2(mtd->size) - ilog2(lock_len);
> >> >>>>>>>>> -     val = mask - (pow << SR_BP_SHIFT);
> >> >>>>>>>>> +
> >> >>>>>>>>> +     if (nor->flags & SNOR_F_HAS_SR_BP3) {
> >> >>>>>>>>> +             val = ilog2(nor->n_sectors) + 1 - pow;
> >> >>>>>>>> 
> >> >>>>>>>> Why do you use a new calculation here? As far as I can
> >> >>>>>>>> see,
> >> >>>>>>>> the
> >> >>>>>>>> method is
> >> >>>>>>>> the same except that is has one bit more. That also
> >> >>>>>>>> raises
> >> >>>>>>>> the
> >> >>>>>>>> question why
> >> >>>>>>>> n_sectors is now needed?
> >> 
> >> Flash devices have variable sector size, 64KB, 128KB or 256KB... While
> >> mapping of number of sectors locked to BP bits is dependent on rules 1
> >> to 3 you mentioned below, the size or area of flash protected depends
> >> on
> >> sector size.
> >> 
> >> So, the current formula in spi-nor.c (ignoring TB and other
> >> boilerplate):
> >> 
> >> pow = ilog2(mtd->size) - ilog2(lock_len);
> >> val = mask - (pow << shift);
> >> 
> >> This works only for devices with 64KB sector size as 8MB flash with
> >> 64KB
> >> sector size would have 128 sectors (BP0-2 => 0b111 => 2^7).
> >> 
> >> A more generic formula would be:
> >> 
> >> Find n where 2^(n - 1) = len/sector-size
> >> OR 2^ (n - 1) = len * n_sectors / mtd->size
> >> 
> >> Which solves to:
> >> 
> >> pow = ilog2(mtd->size) - ilog2(lock_len);
> >> val = ilog2(nor->n_sectors) + 1 - pow;
> > 
> > The current mainline locking support is limited. Michael spotted a good
> > improvement, but I think there are still others that we should
> > consider.
> 
> Sure, as I said my patch was just to show, that there is an underlying
> problem
> and that we should not take the 4th BP bit to differentiate between the
> two
> different formulas.

Right, this is the goal.

Let me try to extend the description of the proposal.

> 
> > We should use a single formula, for all the BP cases. How about the
> > following:
> > 
> > bp_slots_available = (bp_mask >> shift) + 1 - 2;

This formula is derived from Michael's patch. 

A slot (to me) is a horizontal line in the Memory protection table. Maybe we 
can find a better/standardized name for this.

So for BP0-2, bp_slots_available = 6, and for BP0-3, bp_slots_available = 14. 
Notice that I stripped the two special cases: lock none and lock all.

> > bp_slots_needed = ilog2(nor->info->n_sectors);

With bp_slots_needed I tried to describe how many slots are needed if the 
protected density for the first slot is at minimum (sector size).

> > 
> > if (bp_slots_needed > bp_slots_available) {
> > 
> >       bp_slot_count = bp_slots_available;
> >       bp_min_slot_size = nor->info->n_sectors <<
> >       
> >               (bp_slots_needed - bp_slots_available);
> 
> mhh, what is the unit of bp_min_slot_size? bytes or sectors? I guess it
> should

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

which is exactly the protected density for the first slot. The protected 
densities of the other slots can be computed by multiplying with powers of 2.

> be bytes, eg for a 8MiB flash it would be 128kiB and for a 16MiB flash
> it would
> be 256kiB (if there are 3 BP bits).
> 
> > } else {
> > 
> >       bp_slot_count = bp_slots_needed;
> >       bp_min_slot_size = mtd->size >> bp_block_count;

typo: s/bp_block_count/bp_slot_count
> 
> this is a complicated way of saying its the size of one sector, isn't
> it?
> can't we use nor->info->sector_size here? Eg.
> 
> 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 = nor->info->sector_size;
> }

you're right, we're in the else case, where the assumption that the minimum 
protected density is sector size is true, we can use directly nor->info-
>sector_size.

> 
> > }
> > 
> > When both can_be_bottom and can_be_top are true, we prefer the top
> > protection,
> > which is incorrect/buggy/sub-optimal. If the received offset is not
> > aligned to
> > one of the start addresses of the bp slots, then we should up/down
> > align the
> > offset to the closest bp slot, depending on TB and which (top or
> > bottom) fits
> > better. Based on the updated offset and length we can compute the lock
> > range,
> > and after that:
> > 
> > n = ilog2(bp_lock_range/bp_min_slot_size) + 1;
> > val = mask - (n << shift);
> 
> btw. we should catch the two special cases:
>   - lock none -> 0 (that was already the case)
>   - lock all -> all BP bits
> 
> The latter is important if "bp_slots_needed < bp_slots_available"
> because there
> are multiple settings for protect all. Most flashes will define any
> remaining
> setting for "protect all", but I've also seen flashes where the
> in-between ones
> were undefined (not mentioned) and only the "all bit set" was protect
> all.

This case is addressed by using bp_slot_count and bp_slots_available. We're in 
the else case from above. From bp_slot_count up to the bp_slots_available, 
those slots are "protect all".

Cheers,
ta


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