linux-kernel - Re: [PATCH] mtd: spi-nor: sst: Factor out common write operation to `sst_nor_write

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Message-ID: <5fe0e312-0844-4de7-8096-eae24361c0a4@prolan.hu>
Date: Wed, 10 Jul 2024 15:35:38 +0200
From: Csókás Bence <csokas.bence@...lan.hu>
To: Pratyush Yadav <pratyush@...nel.org>
CC: <linux-mtd@...ts.infradead.org>, <linux-kernel@...r.kernel.org>, "Tudor
 Ambarus" <tudor.ambarus@...aro.org>, Michael Walle <mwalle@...nel.org>,
	"Miquel Raynal" <miquel.raynal@...tlin.com>, Richard Weinberger
	<richard@....at>, Vignesh Raghavendra <vigneshr@...com>
Subject: Re: [PATCH] mtd: spi-nor: sst: Factor out common write operation to
 `sst_nor_write_data()`

Hi!

On 7/10/24 15:04, Pratyush Yadav wrote:
>> Notes:
>>      RFC: I'm thinking of removing SPINOR_OP_BP in favor of
>>      SPINOR_OP_PP (they have the same value). SPINOR_OP_PP
>>      is the "standard" name for the elementary unit-sized
>>      (1 byte, in the case of NOR) write operation. I find it
>>      confusing to have two names for the same operation,
>>      so in a followup I plan to remove the vendor-specific
>>      name in favor of the standard one.
> 
> Even though the operations have the same opcode, I see them as different
> operations. One is a byte program: it can only write one byte at a time.
> The other is a page program: it can write up to one page (256 bytes
> usually) at a time.
> 
> So I would actually find it more confusing if you use page program in a
> situation where the operation is actually a byte program, and attempting
> to program the whole page will fail.

Yes, SST engineers took some _unconventional_ steps when designing this 
family... However, there are no 256 byte pages in these chips. You 
either program it one byte at a time, or as a sequence of two byte 
values. So, in my eyes, that makes it a Flash where the page size is 1 
byte, and the vendor-specific write is something extra added on (and 
mind you, that's not a page program either, you just feed it an 
*arbitrary* even number of bytes, there really are no pages here at all, 
only erase sectors).

> Not directly related to this patch, but when reviewing this patch I
> noticed another small improvement you can make. [...]
> Here, we do a write disable. Then if a one-byte write is needed, do a
> write enable again, write the data and write disable.
> 
> Do we really need to toggle write enable between these? If not, it can
> be simplified to only do the write disable after all bytes have been
> written.

Honestly, I'm not sure, I was too afraid to touch that part. However, 
from the datasheet of SST25VF040B I presume that if we did not toggle 
it, then the Flash chip would interpret the 0x02 opcode and its argument 
as another 2 bytes of data to write at the end. Byte Program takes 
exactly 1 argument, so it can be followed by another command, but AAI WP 
goes on until ~CS goes high.

 > Reviewed-by: Pratyush Yadav <pratyush@...nel.org>

Thanks!

Bence