RE: [PATCH v2 1/3] mtd: spi-nor: always respect write-protect input

[Yong Qin <Yong.Qin@xxxxxxxxxxx>]

Hi Tudor,

Good question.

The WP# function is not available when the Quad mode is enabled (CR[1]=1). The WP# function is replaced by IO2 for input and output during Quad mode. With that said, if CR[1] is set to 1 (Quad mode enabled), the registers will not be protected.

Technically default SRWD bit can be set as either 0 or 1. However, as most customers (applications) don't use this feature to protect registers, the default SRWD bit set to 0 might be a better choice, and reserve the option to change to 1 for the applications do need this feature.

Thanks,
Yong

-----Original Message-----
From: Tudor.Ambarus@xxxxxxxxxxxxx <Tudor.Ambarus@xxxxxxxxxxxxx>
Sent: Tuesday, March 12, 2019 5:30 AM
To: Yong Qin <Yong.Qin@xxxxxxxxxxx>; jonas@xxxxxxxxxxx; James Tomasetta <James.Tomasetta@xxxxxxxxxxx>
Cc: linux-mtd@xxxxxxxxxxxxxxxxxxx; bbrezillon@xxxxxxxxxx; richard@xxxxxx; marek.vasut@xxxxxxxxx; computersforpeace@xxxxxxxxx; dwmw2@xxxxxxxxxxxxx
Subject: Re: [PATCH v2 1/3] mtd: spi-nor: always respect write-protect input

Hi, Yong,

Thank you for the explanation. There are still few things to clarify.

On 03/11/2019 10:14 PM, Yong Qin wrote:
> SRWD bit (along with WP#) provides a way to protect Status and Configuration Registers from been modified unintendedly or by a malicious actor.
>
> By default, SRWD bit is 0, which means no protection on registers alternations. Registers can be modified easily by WRR command. (this is most of the application use cases).
>
> If set SRWD bit to 1, then when WP# is driven low during WRR command, WRR command will be ignored and Registers can't be modified. This provides a way to protect Registers, meanwhile still reserve the capability to modify Registers when necessary by driving WP# to high during WRR command.

Does the SRWD bit protect the Status and Configuration Register bits even when in Quad Mode? WP# function is not available in Quad mode. How can one release this protection when in Quad Mode and SRWD set to 1?

If SRWD bit is ignored in Quad Mode, then why didn't Cypress enable Status and Configuration Register bits protection by default? I.e., remove SRWD bit from SR1, make BIT(7) a NOP, and consider the Status and Configuration Register bits protection enabled by default when not in Quad Mode.

Cheers,
ta

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