Re: [PATCH v3 6/6] stm class: ftrace: use different channel accroding to CPU

[Tingwei Zhang]

On Fri, Sep 18, 2020 at 08:45:52PM +0800, Alexander Shishkin wrote:
> Tingwei Zhang <tingwei@xxxxxxxxxxxxxx> writes:
> 
> > @@ -63,6 +65,7 @@ static int __init stm_ftrace_init(void)
> >  {
> >  	int ret;
> >  
> > +	stm_ftrace.data.nr_chans = num_possible_cpus();
> 
> Not a problem with this patch necesarily, but this made me realize that
> .nr_chans may be larger than:
> 
>  (1) what the policy permits,
>  (2) what the stm device can handle.
> 
> While (1) the user can fix in the policy, they won't be able to fix (2),
> in which case they won't be able to use stm_ftrace at all. I'm thinking
> if a link-time callback would be good enough.
>

Hi Alex,

I'm not sure if I understand this correct. If the nr_chans requested by
stm_ftrace is larger than policy permits or stm device can handle,
stm_assign_first_policy() returns with error so stm_source_link_add()
will fail. User would notice that when link happens.  There's not much
we can do if resource is not enough.
 
> Another thing is that .nr_chans needs to be a power of 2 at the moment.
> 
I'll change to below.
stm_ftrace.data.nr_chans = roundup_pow_of_two(num_possible_cpus());
> Regards,
> --
> Alex
> 
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