Re: [PATCH v6 2/2] coresight: cti: Add Qualcomm extended CTI support

[Leo Yan <leo.yan@arm.com>]

Hi Mike,

On Mon, Dec 08, 2025 at 02:47:21PM +0000, Mike Leach wrote:

[...]

> > I tested locally and did not see the GCC complaint for this approach.
> > And this is a global structure with about 16KiB (~4K items x
> 
> Which is precisely the issue - why use 16k bytes of space when a pair
> of indexed tables will use 21 x 32bit locations per table -> 168 bytes
> - 100x smaller!
> 
> This space matters little to high end server systems but is much more
> important in smaller embedded systems.

For the concern of performance and footprint, my approach can
avoid any conversion for standard registers, we end up need to
convert registers for non-standard registers anyway.

I understand your concern for using an array for conversion, this is
cost 16KiB memory but this can benefit a bit performance.  It is a
trade-off between memory and speed.  As said, we can use a static
function for register conversion, the side effect is this might cause
more time.

Given the CTI MMIO register access, I don't think an extra branch
instruction (checking the flag) would cause significant panelty,
given the flag is set once at init and never changed afterwards.

> Moreover the table + inline helper is more efficient at extracting the
> correct offset value. The helper is a simple de-reference - whereas
> the helper functions you suggest require the code to make the
> comparison at every register access.
> The "if qcom ..." may be contained in one place in the source code,
> but is called and executed for every access.
> 
> Why add inefficiencies, either in footprint or execution?

This is about how we design a driver that supports both a standard IP
and non-standard implementations.

Because the standard IP is well defined, its register layout should be
the default; it keeps the code simple and makes future CTI extensions
easier.  For non-standard IPs, we only apply the register translations
needed.

TBH, the optimization topic is a bit over design for me now.  The CTI
module is configured once and remains untouched until it is disabled,
so it is not a hot path.

Thanks,
Leo