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Re: gcc ld and bss sections on SDRAM

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On 18/03/2022 08:56, pozz wrote:
> My platform is LPC546xx, Cortex-M4 by NXP, and the build environment is
> based on GCC ARM toolchain (MCUXpresso IDE). However the question is
> generic.
> 
> The default linker script instructs linker to put bss sections (zero
> initialized data) in internal RAM. During startup, before main, bss
> sections, described in a table on Flash, are reset to zero:
> 
> 
> __attribute__ ((section(".after_vectors.init_bss")))
> void bss_init(unsigned int start, unsigned int len) {
>     unsigned int *pulDest = (unsigned int*) start;
>     unsigned int loop;
>     for (loop = 0; loop < len; loop = loop + 4)
>         *pulDest++ = 0;
> }
> 
> extern unsigned int __bss_section_table;
> extern unsigned int __bss_section_table_end;
> 
> void ResetISR(void) {
>     ...
>     // At this point, SectionTableAddr = &__bss_section_table;
>     // Zero fill the bss segment
>     while (SectionTableAddr < &__bss_section_table_end) {
>         ExeAddr = *SectionTableAddr++;
>         SectionLen = *SectionTableAddr++;
>         bss_init(ExeAddr, SectionLen);
>     }
> 
>     ...
>     main();
>     ...
> }
> 
> 
> This works well until I add a new RAM memory section on external bus (it
> is a SDRAM). Suppose I want to allocate a big variable on SDRAM. This
> can be done in different ways (I use some facilities of MCUXpresso IDE).
> 
> Now the startup code is broken, because SDRAM memory can be accessed
> only after it is configured (during main), but the bss initialization is
> done before main.
> 
> I know I can rewrite ResetISR() or bss_init() to avoid accessing
> external bus addresses or I can configure SDRAM during ResetISR() before
> calling bss_init().
> 
> I suspect there's a better and more cleaner way. Noinit sections?

There are several ways to handle this.  You can set up the sdram before
clearing the bss - I think you'll find a weak function called
SytemInitHook or similar, that you can override with your own version.
If you are putting a lot of different stuff into the sdram, this is
probably the best.

You can fiddle with either the linker script or the ResetISR function to
avoid clearing the sdram bss (and copying the sdram initialised data)
during ResetISR, and do it manually after initialising the ram.

Often, however, it is a good idea /not/ to use sdram for bss or data.
Use the internal ram (DTC ram if possible, or OCRAM - depending on the
particular chip) for that.  Put your heap in sdram, along with other big
buffers such as network buffers, FreeRTOS heap, etc.  These can all be
initialised later once the ram is configured.  And you can mark any
large statically allocated arrays or structures with a section attribute
for a "noinit" section of the sdram.

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Thread

gcc ld and bss sections on SDRAM pozz <pozzugno@gmail.com> - 2022-03-18 08:56 +0100
  Re: gcc ld and bss sections on SDRAM David Brown <david.brown@hesbynett.no> - 2022-03-18 11:15 +0100
    Re: gcc ld and bss sections on SDRAM pozz <pozzugno@gmail.com> - 2022-03-18 11:49 +0100
  Re: gcc ld and bss sections on SDRAM Stefan Reuther <stefan.news@arcor.de> - 2022-03-18 17:57 +0100
    Re: gcc ld and bss sections on SDRAM pozz <pozzugno@gmail.com> - 2022-03-19 16:05 +0100

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