[chbot] Volatile struct puzzle

[Robin Gilks]

It is outside .data and explicitly NOT initialised.
Excerpts from the linker file...

/* Specify the memory areas */
MEMORY
{
DTCM (xrw) : ORIGIN = 0x20000000, LENGTH = 128k
RAM (xrw) : ORIGIN = 0x20020000, LENGTH = 368K
BOOTRAM (xrw) : ORIGIN = 0x2007C000, LENGTH = 16K
FLASH (rx) : ORIGIN = 0x8000000, LENGTH = 2048K
}

/* used by audio buffers and JPEG h/w */
.MCU_Data_section 0x20000000 (NOLOAD) : { *(.MCU_Data_section) } >DTCM

The DTCM memory is by default available to all internal DMA buses (so saves
playing with the cache setup using 'HAL_MPU_ConfigRegion()'.


On Sat, Feb 5, 2022 at 6:47 PM Andrew Dachs <dachsa492 at gmail.com> wrote:

> Hi Robin,
> Is your special area of memory inside .data and part of the initialisation
> on startup?
> Andy
>
>
>
> Sent from my iPhone
>
> On 5/02/2022, at 5:42 PM, Robin Gilks <gb7ipd at gmail.com> wrote:
>
> 
> Hmmm - thanks Mark, I  hadn't thought of packing issues.
> It's not like I'm trying a union to another struct so positioning would be
> critical. That being the case I think I'll keep that thought on the back
> burner for now..
>
> Cheers
>
>
> On Sat, Feb 5, 2022 at 4:10 PM Mark Atherton <markaren1 at xtra.co.nz> wrote:
>
>> I have seen issues with data segments, where large chunks (u32, u8*)
>> follow small chunks (u8).
>>
>> Try changing
>>
>>  > uint8_t State;
>>  > uint8_t *DataBuffer;
>>  > uint32_t DataBufferSize;
>>
>> to
>>
>>  > uint32_t DataBufferSize;
>>  > uint8_t *DataBuffer;
>>  > uint8_t State;
>>
>> -Mark
>>
>>
>>
>>
>> On 5/02/2022 1:50 PM, Robin Gilks wrote:
>> > Some may recall that a year or 2 ago I had great problems getting the
>> > hardware JPEG encode to work on an STM32 processor. It turned out that
>> > the memory cache attributes manifested as a race condition between DMA
>> > and CPU access.
>> > The problem has just reappeared as a result of updating
>> > from arm-none-eabi-gcc-9.2.1-1.1 to arm-none-eabi-gcc-10.3.1-2.3.
>> >
>> > After a few pokes at the code I've narrowed the issue down to whether a
>> > volatile struct element is initialized statically  or at run time.
>> > Interestingly the issue is inverted between compiler versions
>> > The buffer referenced by the structure element 'DataBuffer'  is defined
>> > and instantiated in a reserved section of memory that has the correct
>> > cache attributes
>> >
>> > typedef struct
>> > {
>> > uint8_t State;
>> > uint8_t *DataBuffer;
>> > uint32_t DataBufferSize;
>> > }JPEG_Data_BufferTypeDef;
>> > uint8_t
>> >
>> JPEG_Data_InBuffer[CHUNK_SIZE_IN]__attribute__((section(".MCU_Data_section")));
>> > original code - works with gcc-9; fails with gcc-10:
>> > volatile JPEG_Data_BufferTypeDef Jpeg_IN_BufferTab =
>> {JPEG_BUFFER_EMPTY,
>> > JPEG_Data_InBuffer, 0};
>> > current code fails with gcc-9 works with gcc-10:
>> > volatileJPEG_Data_BufferTypeDefJpeg_IN_BufferTab= {0, 0, 0};
>> > ...
>> > Jpeg_IN_BufferTab.DataBufferSize = 0;
>> > Jpeg_IN_BufferTab.State = JPEG_BUFFER_EMPTY;
>> > Jpeg_IN_BufferTab.DataBuffer = JPEG_Data_InBuffer;
>> >
>> > Note that NO other changes (apart from the compiler version) were
>> made!!
>> > I'm at a total loss as to what is going on, hopefully someone can shine
>> > a light ;)
>> >
>> > --
>> > Robin Gilks
>> >
>> >
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