RTOS/PROCESSOR-SDK-AM437X: Atomic use of Queue_get

Mads Lind Christiansen

Part Number: PROCESSOR-SDK-AM437X
Other Parts Discussed in Thread: SYSBIOS

Tool/software: TI-RTOS

I would like to use the SYS/BIOS Queue_get() function for taking out elements of my free queue. It has been a while (more than 2 years) since I last used the Queue functionality.

From here (software-dl.ti.com/.../Queue.html

I can see that Queue_get() works atomically, which is what I need. It seems though that if the queue is empty it will return non-NULL and the manual states to uses Queue_empty() to check if there are any elements in the queue. This is really bad design (well at least in my case). Now I will have to do my own protection of the queue to make sure that I atomically can take out elements of the queue and be sure that I do actually get a valid element.
E.g.
1. Protect access to queue

2. Check queue is not empty

3. Get element from queue

4. Unprotect access to queue

I believe in the earlier days (DSP/BIOS) you would get a pointer to your queue handle, if the queue is empty?

Does this still apply and if so can I use this safely (considering future revisions of SYS/BIOS)?

/Mads

over 7 years ago

0 Biser Gatchev-XID over 7 years ago

TI__Guru**** 393215 points

The RTOS team have been notified. They will respond here.

0 Mads Lind Christiansen over 7 years ago in reply to Biser Gatchev-XID

Expert 1130 points

Any update on this?

0 lding over 7 years ago in reply to Mads Lind Christiansen

TI__Guru* 95265 points

Hi,

This is more of a SYSBIOS question. Do you have any example code on AM437x how this queue_get() was used in SYSBIOS content? Thanks!

Regards, Eric

0 Mads Lind Christiansen over 7 years ago in reply to lding

Expert 1130 points

Hi Eric

I do the following in my code, and it works.

This feature seems undocumentet by looking at your online SYS/BIOS documentation (see the link in my first post).

    // Get free queue element
    pQ = Queue_get(qHandle);

    // If returned element points to handle, queue is empty!
    if (pQ == (void*) qHandle)
        return STAT_ERROR;

But by looking at old DSP/BIOS documentation this is a documented feature of QUE_get() (which I assume is the same as Queue_get()). See spru625e.pdf page 2-272.

http://software-dl.ti.com/targetcontent/legacy_files/targetcontent/bios/bios_5_20/sabios-e23/docs/packages/ti/bios/doc/spru625e.pdf

This is what the old documentation says:

Calling QUE_get with an empty queue returns the queue itself. This provides a means for using a single atomic action to check if a queue is empty, and to remove and return the first element if it is not empty:

if ((QUE_Handle)(elem = QUE_get(q)) != q) ` process elem `

I assume this also applies to the non atomic version - Queue_dequeue().

Regards,

Mads

0 Mads Lind Christiansen over 7 years ago in reply to Mads Lind Christiansen

Expert 1130 points

Anything on this?

0 ToddMullanix over 7 years ago in reply to Mads Lind Christiansen

TI__Guru* 96960 points

Hi Mads,

The documentation is misleading (at best). You are correct that SYS/BIOS' Queue is basically the same as DSP/BIOS' QUE.

The atomic Queue_get always returns a non-NULL Ptr (actually a Queue_Elem *). If the queue was empty, the return value can be checked against the Queue_Object. If they are the same, the Queue is empty (or you've looped around the tail and are back at the beginning if you are traversing the queue).

The same is true with Queue_dequeue except it is not atomic. This API is useful in cases where you already have interrupts disabled so you don't need the extra atomic code in the API.

I'm open an enhancement request to clean-up the docs.

Todd

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RTOS/PROCESSOR-SDK-AM437X: Atomic use of Queue_get