[PATCH 2/2] mtd: spi-nor: fix mr25h* definitions

[Cyrille Pitchen]

Le 18/01/2017 à 16:57, Marek Vasut a écrit :
> On 01/17/2017 03:51 PM, Bastian Stender wrote:
>> Hi Marek,
> 
> Hi!
> 
>> On 01/16/2017 05:07 PM, Marek Vasut wrote:
>>> On 01/16/2017 04:12 PM, Bastian Stender wrote:
>>>> The previous definitions for mr25h* chips lead to unnecessary limits
>>>> regarding writesize and eraseblocksize for MRAM. MRAM is random access
>>>> memory rather than page, sector, or block organized memory.
>>>>
>>>> Instead of 1 sector with 512KB the mr25h40 MRAM chip should be defined
>>>> as 512K sectors with 1 byte each. This makes it writesize and
>>>> eraseblocksize aligned and reflects MRAM functionality properly.
>>>
>>> Wouldn't this degrade the performance ? ie. if you write the flash, it
>>> will send PP opcode + 1 byte in a loop, right ?
>>
>> I don't think so, info->sector_size and info->n_sectors are used for
>> calculating mtd->size and setting mtd->erasesize only. mtd->erasesize
>> should be never used when SPI_NOR_NO_ERASE is set in info->flags as
>> Cyrille pointed out.
> 
> Hmmm, does that answer my second question ?
> 

Your second question is about Page Program with a single data byte, right?

Once in spi-nor.c, spi_nor_write() checks nor->page_size (=
info->page_size) to limit the amount of data written into the memory in a
single SPI command: spi_nor_write() itself doesn't check nor->mtd.erasesize
(= info->sector_size) but it doesn't mean that upper mtd layers had not
clamped the 'len' parameter before based on the mtd->erasesize limit.

Also I don't know whether the upper mtd layers expect cases where
mtd->writebufsize (= nor->page_size == 256) > mtd->erasesize (=
info->sector_size == 1).

If we also changed the page size to 1, we would keep the relation (page
size <= erase size) but for sure, spi-nor would also send many Page Program
commands with single data byte, hence a huge performance loss.

That's why I think we should take time to study to be sure the proposed
modification won't introduce any unwanted side effect.

Best regards,

Cyrille