EFL driver: don't write FFs

[faisal]

STM32 EFL drivers shouldn't to write to flash if the entire page the user wants to write contain 1s. On the STM32, if you write to a page - even if it is all 1s and did not change the state of flash, it generates a page write error on a subsequent write.

On some devices, the flash memory will allow you to 'write' multiple times to the same page as long as you're writing all 1s (or in some devices it will always allow you to overwrite - with the caveat that you are aware that you can only clear bits and not set them).

This small change will allow for cleaner APIs to be built on top of the EFL, which need to do multiple read-modify-writes on embedded flash while minimizing the number of erase cycles required. Without this, when doing multiple read-modify-write operations you would have to:
1. Erase the entire sector before each RMW operation.
2. Erase the sector once, but keep track of which pages contain all 1s when writing - this results in more overhead to inspect memory and calls to EFL API.
3. Other shenanigans I haven't thought of.

Code: Select all

diff --git c/os/hal/ports/STM32/STM32L4xx/hal_efl_lld.c w/os/hal/ports/STM32/STM32L4xx/hal_efl_lld.c
index 6ea46f8b1..009336c74 100644
--- c/os/hal/ports/STM32/STM32L4xx/hal_efl_lld.c
+++ w/os/hal/ports/STM32/STM32L4xx/hal_efl_lld.c
@@ -313,6 +313,15 @@ flash_error_t efl_lld_program(void *instance, flash_offset_t offset,
     }
     while ((n > 0U) & ((offset & STM32_FLASH_LINE_MASK) != 0U));
 
+    if (efl_lld_descriptor.attributes & FLASH_ATTR_ERASED_IS_ONE)
+    {
+        /* Don't program page if data is all 1s */
+        if ((line.w[0] == 0xFFFFFFFF) && (line.w[1] == 0xFFFFFFFF))
+        {
+            continue;
+        }
+    }
+
     /* Programming line.*/
     address[0] = line.w[0];
     address[1] = line.w[1];

[Giovanni]

Hi,

It is because ECC/parity mechanisms, there is a difference between an erased page (the ECC is all ones too) and a written page with ECC not all at ones.

The ECC information (non rewrite-ability) is in the flash descriptor, it should be the application responsibility to not perform rewrites on such memories unless the purpose IS to create a faulty page, as a guard or for testing for example. In my opinion writing all ones should not be prevented because it activates the ECC part of the line so it actually does something and it is meant to. Upper layers can skip writing all ones if this is the desired behaviour.

Giovanni