Memory allocation form HWI/SWI context

Sandhya NS

Prodigy 170 points

Other Parts Discussed in Thread: TM4C1294KCPDT

Hi,

Can we use "std::new" call inside HWI/SWI context for allocating memory?

Regards,

Sandhya

over 6 years ago

0 YiKai Chen over 6 years ago

Guru 735695 points

No, you cannot.

0 Sandhya NS over 6 years ago in reply to YiKai Chen

Prodigy 170 points

Thanks for the response.

But I haven't get the reason why "std::new" is not allowed in HWI/SWI context.

I am working with TM4C1294KCPDT controller and using TI-RTOS 2.16 version. For the project purpose, I want to create a hardware timer and in the timer

callback need to allocate memory for updating some buffer and the same needs to be send it via Mailbox_post().

Whether this memory allocation will create any issue or not ?

Regards,

sandhya

0 ToddMullanix over 6 years ago in reply to Sandhya NS

TI__Guru* 96960 points

Sandhya,

TI-RTOS by defaults supplies the "malloc" function. It calls HeapMem_alloc (again by default). HeapMem uses semaphores (actually a Gate that is based on a semaphore) by default. Since "new" calls "malloc", your Hwi (or Swi) could be calling Semaphore_pend with a non-zero timeout, which is a no-no.

You can change the behavior of malloc by supplying a different default heap that don't use a Gate (e.g. use HeapBuf or HeapMultiBuf), but then you have to have a good idea on what the bucket sizes should be for the heap.

You can use a Hwi friendly gate (e.g. GateHwi) within HeapMem, but realize this will impact interrupt latency since internal traversals of the HeapMem freelist will be done with interrupts disabled.

Todd