gcc ld and bss sections on SDRAM

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?

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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.

Reply to
David Brown

ResetISR() is the first call after a reset.

__attribute__ ((section(".after_vectors"))) void ResetISR(void) {

// // Copy the data sections from flash to SRAM. // unsigned int LoadAddr, ExeAddr, SectionLen; unsigned int *SectionTableAddr;

// Load base address of Global Section Table SectionTableAddr = &__data_section_table;

// Copy the data sections from flash to SRAM. while (SectionTableAddr < &__data_section_table_end) { LoadAddr = *SectionTableAddr++; ExeAddr = *SectionTableAddr++; SectionLen = *SectionTableAddr++; data_init(LoadAddr, ExeAddr, SectionLen); } // 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); }

#if defined (__USE_CMSIS) || defined (__USE_LPCOPEN) SystemInit(); #endif

#if defined (__cplusplus) // // Call C++ library initialisation // __libc_init_array(); #endif

#if defined (__REDLIB__) // Call the Redlib library, which in turn calls main() __main() ; #else main(); #endif

// // main() shouldn't return, but if it does, we'll just enter an infinite loop // while (1) { ; } }

There isn't any weak function call, anyway I can add my own.

Yes, this is another possibility.

You didn't mention NOLOAD (noinit). It seems to me another good approach for my problem.

I have some images saved in a SDCard and I have to show them on the display in some cases. I have plenty of SDRAM for other purposes (framebuffer, for example) so I thought, why read images from SDCard every time I have to show them?

At startup, I copy images from SDCard to SDRAM and later I can access images directly from SDRAM. The maximum size of all images is 2MB and I have this free space on SDRAM.

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Am 18.03.2022 um 08:56 schrieb pozz:

When I did bootloaders, I did the SDRAM initialisation there, before the application starts. This also means there's a nice place to deal with board varieties, i.e. if I have a board variant with a different SDRAM chip, I only have to change the bootloader. And it allows loading parts of the application code into SDRAM, if that is relevant for you.

Otherwise, I'd probably use a noinit section.


Reply to
Stefan Reuther

Il 18/03/2022 17:57, Stefan Reuther ha scritto:

Yes, you're right, bootloader is another good place to configure hw and next launch application.

Thank you for your suggestion.

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