On 15/08/13 12:32, Tomasz Figa wrote:
> Hi Sudeep,
>
> I don't like this constant DT parsing every time a node of given CPU is
> required, but I believe it was correctly discussed with people that are
> more into CPU topologies and similar things than me. (My idea would be to
> make a lookup array with logical ID to struct device_node * mapping.)
>
Yes that's the idea, see the last paragraph in the commit log.
> Let me just review this from DT parsing perspective.
>
> On Monday 22 of July 2013 12:32:12 Sudeep KarkadaNagesha wrote:
>> From: Sudeep KarkadaNagesha <sudeep.karkadanagesha@xxxxxxx>
>>
>> Currently different drivers requiring to access cpu device node are
>> parsing the device tree themselves. Since the ordering in the DT need
>> not match the logical cpu ordering, the parsing logic needs to consider
>> that. However, this has resulted in lots of code duplication and in some
>> cases even incorrect logic.
>>
>> It's better to consolidate them by adding support for getting cpu
>> device node for a given logical cpu index in DT core library. However
>> logical to physical index mapping can be architecture specific.
>>
>> This patch adds of_get_cpu_node to retrieve a cpu device node for a
>> given logical cpu index. The default matching of the physical id to the
>> logical cpu index can be overridden by architecture specific code.
>>
>> It is recommended to use these helper function only in pre-SMP/early
>> initialisation stages to retrieve CPU device node pointers in logical
>> ordering. Once the cpu devices are registered, it can be retrieved
>> easily from cpu device of_node which avoids unnecessary parsing and
>> matching.
>>
Here we go, do I need to emphasis this recommendation more ?
>> Acked-by: Rob Herring <rob.herring@xxxxxxxxxxx>
>> Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@xxxxxxx>
>> ---
>> drivers/of/base.c | 72
>> ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>> include/linux/of.h | 6 +++++
>> 2 files changed, 78 insertions(+)
>>
>> diff --git a/drivers/of/base.c b/drivers/of/base.c
>> index 5c54279..1e690bf 100644
>> --- a/drivers/of/base.c
>> +++ b/drivers/of/base.c
>> @@ -230,6 +230,78 @@ const void *of_get_property(const struct
>> device_node *np, const char *name, }
>> EXPORT_SYMBOL(of_get_property);
>>
>> +/*
>> + * arch_match_cpu_phys_id - Match the given logical CPU and physical id
>> + *
>> + * @cpu: logical index of a cpu
>> + * @phys_id: physical identifier of a cpu
>> + *
>> + * CPU logical to physical index mapping is architecture specific.
>> + * However this __weak function provides a default match of physical
>> + * id to logical cpu index.
>> + *
>> + * Returns true if the physical identifier and the logical index
>> correspond + * to the same cpu, false otherwise.
>> + */
>> +bool __weak arch_match_cpu_phys_id(int cpu, u64 phys_id)
>> +{
>> + return (u32)phys_id == cpu;
>> +}
>> +
>> +/**
>> + * of_get_cpu_node - Get device node associated with the given logical
>> CPU + *
>> + * @cpu: CPU number(logical index) for which device node is required
>> + *
>> + * The main purpose of this function is to retrieve the device node for
>> the + * given logical CPU index. It should be used to intialize the
>> of_node in + * cpu device. Once of_node in cpu device is populated, all
>> the further + * references can use that instead.
>> + *
>> + * CPU logical to physical index mapping is architecture specific and
>> is built + * before booting secondary cores. This function uses
>> arch_match_cpu_phys_id + * which can be overridden by architecture
>> specific implementation. + *
>> + * Returns a node pointer for the logical cpu if found, else NULL.
>> + */
>> +struct device_node *of_get_cpu_node(int cpu)
>> +{
>> + struct device_node *cpun, *cpus;
>> + const __be32 *cell;
>> + u64 hwid;
>> + int ac, prop_len;
>> +
>> + cpus = of_find_node_by_path("/cpus");
>> + if (!cpus) {
>> + pr_warn("Missing cpus node, bailing out\n");
>> + return NULL;
>> + }
>> +
>> + if (of_property_read_u32(cpus, "#address-cells", &ac)) {
>> + pr_warn("%s: missing #address-cells\n", cpus->full_name);
>> + ac = of_n_addr_cells(cpus);
>
> I'm not sure this fallback is appropriate. According to ePAPR:
>
> "The #address-cells and #size-cells properties are not inherited from
> ancestors in the device tree. They shall be explicitly defined."
>
> In addition:
>
> If missing, a client program should assume a default value of 2 for
> #address-cells, and a value of 1 for #size-cells.
>
> This also leaves in question the correctness of of_n_addr_cells() and
> of_n_size_cells().
>
Yes agreed. We can discus that and fix it separately as it might affect
multiple users.
>> + }
>> +
>> + for_each_child_of_node(cpus, cpun) {
>> + if (of_node_cmp(cpun->type, "cpu"))
>> + continue;
>> + cell = of_get_property(cpun, "reg", &prop_len);
>> + if (!cell) {
>> + pr_warn("%s: missing reg property\n", cpun-
>> full_name);
>> + continue;
>> + }
>> + prop_len /= sizeof(*cell);
>> + while (prop_len) {
>> + hwid = of_read_number(cell, ac);
>> + prop_len -= ac;
>> + if (arch_match_cpu_phys_id(cpu, hwid))
>> + return cpun;
>
> This is a nice potential infinite loop. Consider following example:
>
Good point, but based on the other discussion recently with PPC guys to
support thread ids, I have changed this loop differently, it should not
have this issue.
Regards,
Sudeep
> cpus {
> #address-cells = <2>; /* A typo. Should be 1. */
> #size-cells = <0>;
>
> cpu@0 {
> /* ... */
> reg = <0>;
> };
> };
>
> In this case prop_len will start with 1, while ac will be 2. After first
> iteration of the loop (when the phys id doesn't match) you will end up
> with prop_len = -1 and each iteration will decrement it even more.
>
> By the way, I'm not sure why the whole loop is here. IMHO it should be
> something like:
>
> if (prop_len != ac) {
> pr_warn(...); // or whatever
> continue;
> }
>
> hwid = of_read_number(cell, ac);
> // ...
>
> Best regards,
> Tomasz
>
--
To unsubscribe from this list: send the line "unsubscribe devicetree" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
