Re: [dpdk-dev] [PATCH] common/mlx5: fix CPU detection for PCI relaxed ordering

[Thomas Monjalon <thomas@monjalon.net>]

19/07/2020 13:41, Matan Azrad:
> 
> From: Thomas Monjalon:
> > 19/07/2020 12:56, Matan Azrad:
> > >
> > > From: Thomas Monjalon
> > > > The detection of the CPU was done in a constructor and shared in a
> > > > global variable.
> > > >
> > > > This variable may not be visible in the net PMD because it was not
> > > > exported as part of the .map file.
> > >
> > > Can you explain exactly when it is not visible?
> > 
> > I depends on linker options.
> > 
> > > > It is fixed by exporting a function, which is cleaner than a variable.
> > >
> > > Can you explain why?
> > > We have classic example - rte_eth_devices.
> > 
> > There is more control and more abstraction in functions, it can provide futre-
> > proof abstraction.
> 
> Also variable have more abstraction - struct.
> In future, if it will be needed, we can change it.
> 
> > We should not export variables at all,
> > it is a basic rule of writing API.
> 
> It is variable which is depended only in the running CPU - almost like compile time condition,
> so it is not regular case.
> I think it makes sense also to use a singleton variable as internal API.
> 
> > Having a bad example in ethdev doesn't mean we should follow it.
> 
> If ethdev rte_eth_devices is bad API, Are you going to change it?

No, we avoid changing API.


> > > > By checking the CPU only at the first call of the function, doing
> > > > the check in a constructor becomes useless.
> > >
> > > Yes, but why not to do it in constructor? this variable is initialized only once
> > and doesn't depend in any parameter.
> > 
> > Constructor must remain minimal.
> > If constructor can be avoided, it must be.
> > This is a golden rule.
> 
> The cpu detection is a fast code.
> 
> Using constructor here makes sense:
> 1. we need only one initialization for all the program.
> 2. no need to take care of multithreading on the single initialization (are your code thread safe?).

I don't see what could be the issue.

> 3. no parameters are required.
> 
> > > > Note: the priority of the constructor was probably irrelevant.
> > 
> > No comment about the constructor priority which was set as LOG for no good
> > reason, proving that this code was not well reviewed?
> 
> I guess  you mean that comment is missing - you right.

No I mean this constructor is declared with LOG priority,
but it is not doing any log registration.

> We want to be sure that the variable is ready before any usage of it in the drivers (even in driver contractors).

It is not used by other constructors.
And avoiding constructor dependencies is exactly why we avoid using constructors at all.


> > > > At the same time, the comments are reworded or dropped if useless.
> > > >
> > > > Fixes: 4c204fe5e5d2 ("common/mlx5: disable relaxed ordering in
> > > > unsuitable
> > > > CPUs")
> > > > Cc: shirik@mellanox.com
> > > > Cc: stable@dpdk.org
> > > >
> > > > Signed-off-by: Thomas Monjalon <thomas@monjalon.net>