linux-kernel - Re: [PATCH 0/3] mtd: Make sure UBIFS does not do multi-pass page programming on flashes that don't support it

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Date:   Tue, 3 Nov 2020 18:15:29 +0530
From:   Pratyush Yadav <p.yadav@...com>
To:     Vignesh Raghavendra <vigneshr@...com>
CC:     Richard Weinberger <richard.weinberger@...il.com>,
        Tudor Ambarus <tudor.ambarus@...rochip.com>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        <linux-mtd@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] mtd: Make sure UBIFS does not do multi-pass page
 programming on flashes that don't support it

On 03/11/20 05:05PM, Vignesh Raghavendra wrote:
> 
> 
> On 11/1/20 3:14 AM, Richard Weinberger wrote:
> > On Tue, Oct 27, 2020 at 12:24 PM Pratyush Yadav <p.yadav@...com> wrote:
> >>> [0] https://lore.kernel.org/linux-mtd/20201005153138.6437-1-p.yadav@ti.com/
> >>
> >> Ping. Any comments on the series?
> > 
> > From the UBIFS point of view I'd like to avoid as many device specific
> > settings as possible.
> > We check already for NOR flash, checking for NOR *and* SPI_NOR_NO_MULTI_PASS_PP
> > feels a bit clumsy.
> > 
> > Tudor, what do you think about SPI_NOR_NO_MULTI_PASS_PP?
> > This kind of NOR seems to be a little NAND'ish. Maybe we can hide this detail
> > in the mtd framework?
> > 
> 
> Agree with Richard. I don't see need for SPI_NOR_NO_MULTI_PASS_PP. From
> MTD point of view setting mtd->writesize to be equal to pagesize should
> be enough. Its upto clients of MTD devices to ensure there is no multi
> pass programming within a "writesize" block.

That is what I initially thought too but then I realized that multi-pass 
programming is completely different from page-size programming. Instead 
of writing 4 bytes twice, you can zero out the entire page in one single 
operation. You would be compliant with the write size requirement but 
you still do multi-pass programming because you did not erase the page 
before this operation.

It is also not completely correct to say the Cypress S28 flash has a 
write size of 256. You _can_ write one byte if you want. You just can't 
write to that page again without erasing it first. For example, if a 
file system only wants to write 128 bytes on a page, it can do so 
without having to write the whole page. It just needs to make sure it 
doesn't write to it again without erasing first.

nor_erase_prepare() was written to handle quirks of some specific 
devices. Not every device starts filling zeroes from the end of a page. 
So we have device-specific code in UBIFS already. You will obviously 
need device-specific settings to have control over that code.

One might argue that we should move nor_erase_prepare() out of UBIFS. 
But requiring a flash to start erasing from the start of the page is a 
UBIFS-specific requirement. Other users of a flash might not care about 
it at all.

And so we have ourselves a bit of a conundrum. Adding 
SPI_NOR_NO_MULTI_PASS_PP is IMHO the least disruptive answer. If the 
file system wants to do multi-pass page programming on NOR flashes, how 
else do we tell it not to do it for this specific flash?

> If this is not clear in the current documentation of struct mtd, then
> that can be updated.

-- 
Regards,
Pratyush Yadav
Texas Instruments India