[U-Boot] sf: proposed status register protect

[George McCollister]

On Thu, Oct 6, 2016 at 11:55 AM, Jagan Teki <jagannadh.teki at gmail.com> wrote:
> On Thu, Oct 6, 2016 at 9:21 PM, George McCollister
> <george.mccollister at gmail.com> wrote:
>> On Thu, Oct 6, 2016 at 10:27 AM, Jagan Teki <jagannadh.teki at gmail.com> wrote:
>>> On Thu, Oct 6, 2016 at 8:32 PM, George McCollister
>>> <george.mccollister at gmail.com> wrote:
>>>> I'm looking into adding a sub-command to sf to protect status
>>>> registers on Winbond SPI flash parts via the status register protect
>>>> bits SRP0, SRP1. I propose the sub-command be named sr-protect.
>>>>
>>>> My use case is simple, I want to make the SPI flash read-only when the
>>>> /WP pin is low. To do this I must do 'sf protect lock 0 $size' then
>>>> set SRP0 to 1 and hold the /WP pin low. Although my use case doesn't
>>>> require it, setting SRP1 to 1 allows either Power Supply Lock-Down or
>>>> One Time Program (if supported by hardware) and I would probably add
>>>> support for these as well.
>>>>
>>>> All of the current Winbond SPI NOR flash part datasheets that I've
>>>> looked at show that the parts support the same locking implemented in
>>>> stm_lock() even though SPI_FLASH_CFI_MFR_WINBOND is not in the list of
>>>> vendors which use the function. I'll change this to make
>>>> SPI_FLASH_CFI_MFR_WINBOND parts use stm_lock(), stm_unlock(),
>>>> stm_is_locked().
>>>
>>> Does stm_lock can't protect the Winbond? how different is in the
>>> process of doing when compared to stm?
>>
>> stm_lock() does work with the Winbond parts. The issue is that you can
>> simply unlock from software (stm_unlock). To make it impossible to
>> write to the flash from software you must use stm_lock() then use the
>> status register protect bits to prevent an unlock.
>
> So, you need even if stm_unlock triggered from cmd flash still lock?
> then what is the point of using unlock?

stm_lock()/stm_unlock() change the block protect bits in the status
register to lock/unlock flash blocks. The status register protect bits
allow changes to the status register to be blocked under certain
conditions (if /WP pin is low, until re-powered or permanently).

> Just describe me what is the diff b/w winbond vs other stm flash in
> case of lock and unlocking.

I think it may be the same but if you have a part number this code is
known to work with I'll get the datasheet and check. The comment above
stm_lock() actually mentions a Winbond part! As far as I can tell
STMicroelectronics no longer manufactures SPI Flash.

>
> thanks!
> --
> Jagan Teki
> Free Software Engineer | www.openedev.com
> U-Boot, Linux | Upstream Maintainer
> Hyderabad, India.