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doc: reference: float: document ARM thread tag recommendation

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This commit adds a short note in reference/kernel/other/float.
The note instructs to apply pre-tagging of ARM threads to
indicate that they indend to use the FP services.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
This commit is contained in:
Ioannis Glaropoulos 2019-06-18 17:54:11 +02:00 • committed by Andrew Boie
commit 0bae5a48d4
1 changed files with 13 additions and 0 deletions
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doc/reference/kernel/other/float.rst
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@ -77,6 +77,19 @@ This means that any thread is allowed to access the floating point registers.
The ARM kernel automatically detects that a given thread is using the floating
point registers the first time the thread accesses them.
Pretag a thread that intends to use the FP registers by
using one of the techniques listed below.
* A statically-created ARM thread can be pretagged by passing the
:c:macro:`K_FP_REGS` option to :c:macro:`K_THREAD_DEFINE`.
* A dynamically-created ARM thread can be pretagged by passing the
:c:macro:`K_FP_REGS` option to :cpp:func:`k_thread_create()`.
Pretagging a thread with the :c:macro:`K_FP_REGS` option instructs the
MPU-based stack protection mechanism to properly configure the size of
the thread's guard region to always guarantee stack overflow detection.
During thread context switching the ARM kernel saves the *callee-saved*
floating point registers, if the switched-out thread has been using them.
Additionally, the *caller-saved* floating point registers are saved on