[chbot] Volatile struct puzzle

Sat Feb 5 12:11:14 GMT 2022

Have you tried sprinkling memory barriers between the CPU access to
the DMA memory and the DMA operation trigger?

On Sat, Feb 5, 2022 at 8:23 PM Andrew Dachs <dachsa492 at gmail.com> wrote:
>
> I see. Could it be that the initialisation code changes are just moving things about in memory rather than root cause? Why is it important that the control struct is volatile or is it really just the contents of DataBuffer that’s being changed?
>
>
>
> Sent from my iPhone
>
> On 5/02/2022, at 7:17 PM, Robin Gilks <gb7ipd at gmail.com> wrote:
>
> 
> 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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>>
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