Re: [dpdk-dev] [PATCH v2 0/3] Allow overriding of build-time constants

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Oct 19, 2020 at 11:04:54PM +0200, Thomas Monjalon wrote:
> 19/10/2020 12:21, Bruce Richardson:
> > On Fri, Oct 16, 2020 at 06:46:12PM +0200, David Marchand wrote:
> > > On Fri, Oct 16, 2020 at 5:56 PM Bruce Richardson
> > > <bruce.richardson@intel.com> wrote:
> > > > > librte_eal.so is indeed built with the 64 value:
> > > > > $ pahole -C rte_mem_config build/install/lib64/librte_eal.so |grep memsegs
> > > > > die__process_function: tag not supported (INVALID)!
> > > > >     struct rte_memseg_list     memsegs[64];          /*   136  8704 */
> > > > >
> > > > >
> > > > > But no trace of the custom value for external applications:
> > > > > $ grep -r RTE_MAX_MEMSEG_LISTS build/install
> > > > > build/install/include/rte_config.h:#ifndef RTE_MAX_MEMSEG_LISTS
> > > > > build/install/include/rte_config.h:#define RTE_MAX_MEMSEG_LISTS 128
> > > > > Binary file build/install/lib64/librte_eal.a matches
> > > > > Binary file build/install/lib64/librte_eal.so.21.0 matches
> > > > >
> > > > > I can see the same using the meson option -Dc_args.
> > > >
> > > > Good point, I had not thought of external apps using these values.
> > > >
> > > > They are mostly for internal use, so maybe its worthwhile looking to not
> > > > have them in a public header file. What do you think? Is it likely that
> > > > apps would be using some of these values, or needs to know the specifics?
> > > 
> > > Some are publicly exposed, like RTE_MEMPOOL_CACHE_MAX_SIZE,
> > > RTE_PKTMBUF_HEADROOM, RTE_ETHDEV_RXTX_CALLBACKS,
> > > For those, either we propagate the overriden value to the installed
> > > rte_config.h or we refuse customisation.
> > > 
> > I'd suggest the first 2 of those should possibly be global meson options.
> 
> How the application is reading the meson options?
> 
The meson options are reflected in the rte_build_config.h file. It's not
automatic, but they should be set there by the config/meson.build file. If
some are missed, they can be added, but I disagree that all meson options
should always be passed through to apps. It makes them part of the API,
perhaps unnecessarily, and therefore harder to change or adjust.
Furthermore why should an app ever need to care if a DPDK build included
the docs, or the kmods?

> > Third should probably not be exposed at all.
> 
> I think everything should be exposed.
> The application may need to know whether a feature is enabled or not,
> and what is a specific tuning value.
> 
> I suspect it is the last blocker for meson adoption.  Now that we removed
> the makefiles, we need to fill this gap urgently.
> 
I really don't see this as a gap. With "make" we struggled with massive
amounts of build config, and we tried to remove as much as we can. While
this is reporting what's there rather than tweaking it, surely many of
these settings are just better as #defines in the individual header files -
if they need to be exposed at all.

/Bruce