Re: [EXT] Re: [PATCH] config: change default cache line size for ARMv8 with meson

[Luca Boccassi <bluca@debian.org>]

On Wed, 2019-01-09 at 16:36 +0000, Pavan Nikhilesh Bhagavatula wrote:
> On Wed, 2019-01-09 at 15:41 +0000, Luca Boccassi wrote:
> > External Email
> > 
> > -------------------------------------------------------------------
> > ---
> > On Wed, 2019-01-09 at 15:34 +0000, Jerin Jacob Kollanukkaran wrote:
> > > > > Please check below thread and patch.
> > > > > 
> > > > > http://mails.dpdk.org/archives/dev/2019-January/122676.html
> > > > > https://patches.dpdk.org/patch/49477/
> > > > > 
> > > > > Debian folks are building like this for the _generic_ image.
> > > > > What ever works for every distros, I am fine with that.
> > > > > 
> > > > > meson configure -Dmachine=default
> > > > > meson build
> > > > > cd build
> > > > > ninja
> > > > > ninja install
> > > > 
> > > > I think we agree on the idea of having different configs
> > > > for unmodified A72 core and generic build working for all.
> > > 
> > > Yes, I agree. config or some scheme to address the generic and
> > > default
> > > usecase.
> > > 
> > > > The remaining bits to discuss are:
> > > > 	- do we want to use the armv8 config for unmodified
> > > > A72?
> > > > 	- what should be the name of the generic config?
> > > 
> > > If all distros following "meson configure -Dmachine=default"
> > > scheme
> > > why not follow that to make generic image. i.e when
> > > machine=default
> > > set then Cache lize size 128B CL specific stuff be kicked in else
> > > it probe the value based on MIDR from sysfs.
> > > 
> 
> When we are not cross-compiling  can read CTR_EL0[1] register
> (DminLine) to detect the cacheline size based on the native machine.
> This method would satisfy all requirements and we need not maintain a
> mapping for cacheline_sizes w.r.t part numbers.
> 
> 
> [1] 
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.100095_00
> 01_02_en/way1382037583047.html
> 
> (newer kernels expose it as a sysfs entry
>  /sys/devices/system/cpu/cpu0/cache/index0/coherency_line_size, we
> can
> use meson run_command to either run a python program with c byte code
> or use cc.run('<program>') to get result directly).

Please, not for the machine=default case - we need a stable, invariant
option that does not depend on the build worker.

-- 
Kind regards,
Luca Boccassi