p_context in uart_callback_args_t

I would like to use the p_context member of the uart_callback_args_t* that the SCI UART HAL passes into my callback function.

A similar question from 2 years ago says "don't do that" without a satisfactory explanation, so I dug deeper into the SSP HAL code. p_context (at least for the SCI UART) definitely does not point to the function that was executing at the time the callback happened; it gets assigned from p_ctrl->p_context just before the callback function is called. Using the debugger, I can verify that p_context points to g_uart0.

The Syngery-generated code declares a const const uart_cfg_t g_uart0_cfg where .p_callback = user_uart0_callback and .p_context = &g_uart0. g_uart0 is a const uart_instance_t that points back to g_uart0_ctrl and g_uart0_cfg. (Most members of structs are omitted because they are irrelevant to this question.)

R_SCI_UartOpen copies the callback pointer from p_cfg (which is const) to p_ctrl (which is mutable)

p_ctrl->p_callback = p_cfg->p_callback;

r_sci_uart_rxi_isr has this sequence for the callback function, once it checks that p_callback is non-NULL

uart_callback_args_t args;
args.channel = channel;
args.data = 0U;
args.p_context = p_ctrl->p_context;
args.event = UART_EVENT_RX_COMPLETE;
p_ctrl->p_callback(&args);

Note that, per the initialization of g_uart0_cfg, p_ctrl->p_context = &g_uart0. I can't find any code where p_context is actually dereferenced; it only seems to get copied into a p_context of another struct that is passed to another function.

Further, the documentation for sci_uart_instance_ctrl_t.p_context says "Pointer to user interrupt context data," (r_sci_uart.h, line 92) which strongly implies that the user can use this pointer to pass context into the callback function.

So, can I use p_context for any pointer I want to, and if so, is it better to set it once in p_cfg (maybe making a non-const copy and replacing the value) or can I safely change it in p_ctrl after the call to R_SCI_UartOpen?

Also, information about my environment:

Board:  S5D9 PK

Device: R7FS5D97E3A01CFC

Toolchain:  GCC ARM Embedded

Toolchain Version:  4.9.3.20150529

SSP Version:  1.5.0-rc.1