[chbot] Volatile struct puzzle

[Andrew Dachs]

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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