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Re: why do we need xtensa-config.h?


Am 07.09.2016 um 18:21 schrieb augustine.sterling@gmail.com:
> On Tue, Sep 6, 2016 at 11:55 PM, Thomas Schwinge
> <thomas@codesourcery.com> wrote:
>> Hi!
>>
>> Neither do I really know anything about Xtensa, nor do I have a lot of
>> experience in these parts of GCC back ends, but:
> 
> There is a lot of background to know here. Unfortunately, I have no
> familiarity with making debian packages, so I'm unfamiliar with that
> side of it.
> 
> First--and perhaps most important--the current method of configuring
> GCC for xtensa targets has worked well for nearly two decades. As far
> as I know, it is rare to encounter problems. Because of that, the bar
> to change it will probably be fairly high to change it.
> 
>> On Tue, 6 Sep 2016 20:42:53 +0200, Oleksij Rempel <linux@rempel-privat.de> wrote:
>>> i'm one of ath9k-htc-firmware developers. Currently i'm looking for the
>>> way to provide this firmware as opensource/free package for debian. Main
>>> problem seems to be the need to patch gcc xtensa-config.h to make it
>>> suitable for our CPU.
>>>
>>> I have fallowing questions:
>>>
>>> do we really need this patch?
>>> https://github.com/qca/open-ath9k-htc-firmware/blob/master/local/patches/gcc.patch
>>
>> That I can't tell.  ;-)
> 
> You need something like that patch, for sure.
> 
>>> Is it possible or welcome to extend gcc to be configurable without
>>> patching it all the time?
>>
>> Yes, I would think.  The macros modified in the above patch to GCC's
>> include/xtensa-config.h file look like these ought to be modifiable with
>> -m* options defined by the Xtensa back end, and you'd then assign
>> specific defaults to a specific CPU variant, and build GCC (or build a
>> multilib) for that configuration.
> 
> Today, there are literally hundreds of variants of the xtensa cpu
> actually realized and in use. Having a list of all those variants and
> their defaults inside gcc would be awkward and unwieldy.
> 
> But--and here's the rub--literally tomorrow, someone could design a
> hundred more that are different from all of the ones already out
> there. There is literally an unlimited number of potential variants,
> each with potentially brand new, never conceived instructions. (Adding
> clever custom instructions is xtensa's raison d'etre.)
> 
> With the current configurability mechanism, supporting all of those
> variants inside gcc (and, in fact, the rest of the gnu-toolchain) is
> simply a matter of using the correct xtensa-config.h for that
> particular variant. If we were to go with the "-m with defaults"
> mechanism, we would need some way of adding the defaults for the new
> variant to gcc.
> 
> But that is patching gcc also, and once you go there, you may as well
> use the original method.
> 
>>
>> This file include/xtensa-config.h is #included in
>> gcc/config/xtensa/xtensa.h and libgcc/config/xtensa/crti.S,
> 
> Note that "-m" options can't change the instructions in crti.S and
> lib?funcs.S, but macros can and do.
> 
> 
> 
> On the debian packaging side. Forgive me for my ignorance on the
> topic; I don't know that the tool-flow is, or what the requirements
> are. As far as I am aware, this is the first time someone has tried to
> make a debian package for xtensa.

The point is to provide a package for "free" repository. It means, any
one should be able to use "apt-get source package_name", patch it and
recompile it from source.

Right now it would work, but the package scripts should download
toolchain source, patch and compile it and the compile actual firmware.
This is wary high overhead.

This is why i asked my self, why the toolchain can't be modular or
configurable enough to work without patching and recompiling it.

> Anyway, I wouldn't expect patching gcc (or configuring it) for an
> individual package is the right thing. It should probably happen as
> part of some kind of "setup toolchain" step.
> 
> Typically, patching gcc for a xtensa config happens just once
> immediately after designing the processor, or--if you aren't the
> designer yourself--when one starts development for that variant.

after quick look i noticed that:
XSHAL_USE_ABSOLUTE_LITERALS affects TRAMPOLINE_SIZE. This seems to be
hardcoded every where in gcc, so can't be changed dynamically?

XCHAL_HAVE_MUL32_HIGH probably can be changed dynamically.
XCHAL_ICACHE_SIZE and XCHAL_DCACHE_SIZE will enable or disable part of
__xtensa_sync_caches, why not to make it dynamically as command line option?


IMO most of xtensa-config.h can be changed on runtime. Or do i miss some
thing?

> Surely if someone is building this package, they would have already
> built some other software for that particular xtensa target. (Perhaps
> as part of a larger debian build?)
> 
> Also, this package should probably only be built when targeting this
> particular xtensa variant (not xtensa generally). I don't know how one
> restricts this in the debian packaging mechanism.
> 
> Hope this helps, and I'm happy to answer more questions.
> 

currently this patch affects 15 options. Surely it makes no sense to add
support for each xtensa variant. But why not to add 15 runtime
configuration options to avoid patching?

-- 
Regards,
Oleksij

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