On 4/15/25 3:45 PM, Jan Beulich wrote:
On 15.04.2025 15:39, Oleksii Kurochko wrote:
On 4/10/25 5:53 PM, Jan Beulich wrote:
On 08.04.2025 17:57, Oleksii Kurochko wrote:
+{
+ const __be32 *cell;
+ int ac;
+ uint32_t len;
+
+ ac = dt_n_addr_cells(cpun);
+ cell = dt_get_property(cpun, "reg", &len);
+ if ( !cell || !ac || ((sizeof(*cell) * ac * (thread + 1)) > len) )
+ return ~0ULL;
I'm sorry for my lack of DT knowledge, but what's "thread" representing here?
You only pass in 0 below, so it's unclear whether it's what one might expect
(the thread number on a multi-threaded core).
Based on the DT specification alone, the|`reg`| value could refer to either a
CPU or a thread ID:
```
The value of reg is a <prop-encoded-array> that defines a unique CPU/thread id
for
the CPU/threads represented by the CPU node. If a CPU supports more than one
thread
(i.e. multiple streams of execution) the reg prop-erty is an array with 1
element
per thread.
```
My understanding is that the term/thread/ was used in the Linux kernel to cover
both
cases.
When SMT isn't supported, the CPU can be considered to have a single thread.
For example, RISC-V uses the term/hardware thread/ to describe a hart (i.e., a
CPU).
Interestingly, the Linux kernel always uses|thread = 0|.
We could potentially drop this ambiguity and introduce an|ASSERT()| to check
that
the|`reg`| property contains only one entry, representing the HART (CPU) ID:
```
Software can determine the number of threads by dividing the size of reg by
the parent
node’s #address-cells. If `|reg`| has more than one entry, it would simply
SMT support
is required.
```
Does that approach make sense, or should we stick with the current
implementation?
If extra enabling is required to make multi-thread CPUs work, then panic()ing
(not so much ASSERT()ing) may make sense, for the time being. Better would be
if we could use all threads in a system right away.
Actually, this function is ready to be used for multi-thread CPUs. A caller can
request hardware id
by passing `thread` argument (`thread` -> the local thread number to get the
hardware ID for).
So by calling:
dt_get_cpu_hwid(cpu0, 0) -> it will return hardware id of thread 0 of cpu0
dt_get_cpu_hwid(cpu0, 1) -> it will return hardware id of thread 1 of cpu0
...
In our case we assume that SMP isn't supported so that is why it is used only
dt_get_cpu_hwid(cpu0, 0).
If one day, SMP will be enabled then it will be needed to change a callers of
dt_get_cpu_hwid().
I will add a check in the caller.
~ Oleksii
