Re: the cost of EXPORT_SYMBOL_GPL

[Greg KH <greg@kroah.com>]

On Tue, Jun 16, 2020 at 03:18:02PM +0200, Tomek The Messenger wrote:
> Hi
> I am in the middle of implementation of various external kernel modules.
> All kernel modules touch some board specific registers. For example let's
> say we have 3 groups of register areas:
> #define REG1_ADDR (0xFDEA7000)
> #define REG1 _SIZE (0x3C0)
> #define REG2_ADDR (0xFDEC4000)
> #define REG2 _SIZE (0xFF8)
> #define REG3_ADDR (0xFDEA6000)
> #define REG3_SIZE (0x80)
> And let's say we have 5 kernel modules:
> A
> B
> C
> D
> E
> 
> Kernel module A touches only one 32-bit register X from REG1 space and a
> lot of registers from REG2 space.
> Kernel modules B,C,D touches various registers from REG1 and REG2 space but
> not this one which kernel module A touches.
> Kernel module E is the only one who touches registers from REG3 space. No
> other kernel module touches REG3 space.
> 
> If I would implement the code in C++ in user space I would create dynamic
> library for access to REG1,REG2,REG3 register space. Each 32-bit register
> would have his own separate function to read,write to register. And then
> each user space process A-E would be linked against this library during
> compilation.
> When programming in kernel I don't know what I should do.
> Should I implement 6th external kernel module which implements all
> operations on access to registers REG1,REG2,REG3??? There will be a lot of
> EXPORT_SYMBOL_GPL. Or maybe this approach is NOK, I mean it is good but I
> should only place in 6th helper kernel module functions which are used by
> more than one kernel module. So for example functions for getting access to
> REG3 address space or reg X from REG1 space should be not there. They
> should be only in kernel module code A/E. Currently this is code which I
> implemented. In 6th helper kernel module there are only registers which are
> used by more than one kernel module. But I am not sure if it is good
> approach. Maybe there should be there all registers access?
> 
> Could anybody help how to organize code here.

Why is this so many different modules?  Why not just make it one?

thanks,

greg k-h

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