Re: [PATCH v22 07/11] acpi/arm64: Add GTDT table parse driver

[Fu Wei]

Hi Lorenzo,

On 29 March 2017 at 19:33, Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> wrote:
> On Wed, Mar 29, 2017 at 06:48:26PM +0800, Fu Wei wrote:
>> Hi Lorenzo,
>>
>> On 29 March 2017 at 18:21, Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> wrote:
>> > On Wed, Mar 29, 2017 at 05:48:17PM +0800, Fu Wei wrote:
>> >
>> > [...]
>> >
>> >>  * @platform_timer_count: It points to a integer variable which is used
>> >>  *                           for storing the number of platform timers.
>> >>  *                           This pointer could be NULL, if the caller
>> >>  *                           doesn't need this info.
>> >>
>> >> >
>> >> >> + *
>> >> >> + * Return: 0 if success, -EINVAL if error.
>> >> >> + */
>> >> >> +int __init acpi_gtdt_init(struct acpi_table_header *table,
>> >> >> +                       int *platform_timer_count)
>> >> >> +{
>> >> >> +     int ret = 0;
>> >> >> +     int timer_count = 0;
>> >> >> +     void *platform_timer = NULL;
>> >> >> +     struct acpi_table_gtdt *gtdt;
>> >> >> +
>> >> >> +     gtdt = container_of(table, struct acpi_table_gtdt, header);
>> >> >> +     acpi_gtdt_desc.gtdt = gtdt;
>> >> >> +     acpi_gtdt_desc.gtdt_end = (void *)table + table->length;
>> >> >> +
>> >> >> +     if (table->revision < 2)
>> >> >> +             pr_warn("Revision:%d doesn't support Platform Timers.\n",
>> >> >> +                     table->revision);
>> >> >
>> >> > Ok, two points here. First, I am not sure why you should warn if the
>> >> > table revision is < 2, is that a FW bug ? I do not think it is, you
>> >> > can just return 0.
>> >>
>> >> I used pr_debug here before v20, then I got Hanjun's suggestion:
>> >> -------
>> >> GTDT table revision is updated to 2 in ACPI 5.1, we will
>> >> not support ACPI version under 5.1 and disable ACPI in FADT
>> >> parse before this code is called, so if we get revision
>> >> <2 here, I think we need to print warning (we need to keep
>> >> the firmware stick to the spec on ARM64).
>> >> -------
>> >> https://lkml.org/lkml/2017/1/19/82
>> >>
>> >> So I started to use pr_warn.
>> >
>> > Thanks for the explanation, so it is a FW bug and the warning
>> > is granted :) just leave it there.
>> >
>> > Still, please check my comment on acpi_gtdt_init() being called
>> > multiple times on patch 11.
>>
>> Thanks
>>
>> For calling acpi_gtdt_init() twice:
>> (1) 1st time: in early boot(bootmem), for init arch_timer and
>> memory-mapped timer, we initialize the acpi_gtdt_desc.
>> you can see that all the items in this struct are pointer.
>> (2) 2nd time: when system switch from bootmem to slab, all the
>> pointers in the acpi_gtdt_desc are invalid, so we have to
>> re-initialize(re-map) them.
>>
>> I have tested it, if we don't re-initialize  the acpi_gtdt_desc,
>> system will go wrong.
>
> Ok, that's what I feared. My complaint on patch 11 is that:
>
> 1) Stashing the GTDT pointer in acpi_gtdt_desc is not needed to
>    parse SBSA watchdogs

The acpi_gtdt_desc is for sharing the info between acpi_gtdt_init and
acpi_gtdt_c3stop, ;acpi_gtdt_map_ppi
I re-use it in parsing SBSA watchdogs, because I try to re-use acpi_gtdt_init.

> 2) It is not clear at all from the code or the commit log _why_ you
>    need to call acpi_gtdt_init() again (ie technically you don't need
>    to call it - you grab a valid pointer to the table and parse the
>    watchdogs in the _same_ function gtdt_sbsa_gwdt_init())

yes, we can avoid calling acpi_gtdt_init(), do the same thing in
parsing SBSA watchdogs info.
But if we will do the same thing(getting gtdt, platform_timer,
timer_count), why not just re-using the same function?

So my suggestion is that:
Could we re-use acpi_gtdt_init, and a comment at the head of SBSA
watchdogs info parsing function to summarize this issue?
The comment like this

Note: although the global variable acpi_gtdt_desc has been initialized
by acpi_gtdt_init, when we initialized arch_timer. But when we call this
function to get SBSA watchdogs info from GTDT, the system has switch
from bootmem  to slab, so the pointers in acpi_gtdt_desc are dated, we
need to  re-initialize(remap) them. So we call acpi_gtdt_init again here.

Is that OK for you? :-)

>
> I do not think there is much you can do to improve the arch timer gtdt
> interface (and it is too late for that anyway) but in patch 11 it would
> be ideal if you avoid calling acpi_gtdt_init() again, just parse GTDT
> entries and initialize the corresponding watchdogs (ie pointers stashed
> in acpi_gtdt_desc are stale anyway but that's __initdata so I can live
> with that).
>
> You should add comments to summarize this issue so that it can be
> easily understood by anyone maintaining this code, it is not crystal
> clear by reading the code why you need to multiple acpi_gtdt_init()
> calls and that's not a piffling detail.
>
> The ACPI patches are fine with me otherwise, I will complete the
> review shortly.
>
> Thanks !
> Lorenzo



-- 
Best regards,

Fu Wei
Software Engineer
Red Hat