Help Finding "User Tag" Documentation for CRACEN DMA (nRF54L15)

[William Pereira]

I’m working with the CRACEN peripheral on the nRF54L15 and I’m trying to understand the DMA User Tag field used in registers like `CRYPTMSTRDMA.FETCHTAG`.

The documentation mentions this field but doesn’t explain its purpose or valid values, it only says "Fetch User Tag (only used in direct mode)". I’ve checked the nRF54L15 reference manual and CRACEN-related sections, but couldn’t find further details.

Could someone point me to the documentation or provide clarification on what the User Tag represents and how it should be configured?

Thank you.

[Hung Bui]

Hi William, 
The reply I got from the team is that the register is not expected to be used directly by user. The recommendation is "they can just ignore it and use the nrfx driver if they wish to make use of it in a baremetal application: https://github.com/NordicSemiconductor/nrfx/blob/11f57e578c7feea13f21c79ea0efab2630ac68c7/hal/nrf_c…

 The driver uses cryptomaster in scatter/gather mode.

[William Pereira]

Thanks for the clarification. However, I’m still unsure about the User Tag part — even if I’m not supposed to use the register directly, I still need to understand how to fill the dmatag field when building a descriptor for scatter/gather manually.

For example, in C I often see something like:

```
struct sxdesc {
    char *addr;
    struct sxdesc *next;
    uint32_t sz;
    uint32_t dmatag;
};
```
So my question still the same

1. What should `uint32_t dmatag` contain in this context?

Also, I understand the recommendation to use scatter/gather mode, but the documentation note (“only used in direct mode”) implies the tag has some role there. If I want to use direct mode,

2. how should I handle multiple data inputs, and what value should be written to the tag field?

I am looking forward for the answer of both questions! 

Thanks again for the help.

[Hung Bui]

Hi William, 
Could you give us some more info on what you are working on ? 
Are you using our driver ? Or any of our sample ? 

[William Pereira]

Hi Hung,
I’m working in Rust to implement low-level cryptographic functions directly on the nRF54L15.
I generated a PAC from the SVD and I’m building these routines from scratch, without using the Nordic drivers.

My goal is to understand how things work, but the documentation seems to be missing several points, especially regarding how CRACEN operates at the register level, including the meaning of fields like dmatag.

[William Pereira]

If you wanna see, I have a hash working here, but I am still using some magic number because I don't know what to put inside `dmatag`: github.com/.../sha256-scatter-gather-mode.rs

[Hung Bui]

Hi William, 
Unfortunately the documentation for CRACEN is still quite limited. The suggestion from R&D team is that you would have to look at our code to find the info you need for your driver. They pointed to: 

https://github.com/nrfconnect/sdk-nrf/tree/main/subsys/nrf_security/src/drivers/cracen/silexpk

https://github.com/nrfconnect/sdk-nrf/tree/main/subsys/nrf_security/src/drivers/cracen/sxsymcrypt

[William Pereira]

Thanks for checking with the team.

Do you have any estimation or roadmap for when the CRACEN documentation will be updated or expanded? That would help plan our development efforts.

[Hung Bui]

Hi William, 
I did a quick check and it seems that we don't have a clear roadmap on when CRACEN full documentation will be available unfortunately. So the driver code is the best we have for now.