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


Am 08.09.2016 um 18:10 schrieb augustine.sterling@gmail.com:
> On Thu, Sep 8, 2016 at 8:14 AM, Oleksij Rempel <linux@rempel-privat.de> wrote:
>> 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?
> 
> TRAMPOLINE_SIZE is used in quite a few places (so beyond my authority
> to accept patches), but I suspect it would be acceptable to put a
> function behind TRAMPOLINE_SIZE that calculated the value 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?
> 
> Unless we can make it all of what xtensa-config.h provides, we don't
> really solve the problem.
> 
> Also, I'm wary of adding command line options that are required to get
> correct object-code. GCC can't validate the options against the
> hardware it is actually targeting (that's what xtensa-config.h
> actually tells it, so without a correct one, it can't know).
> 
> This also puts some burden on every one who uses it--passing fifteen
> -mfoo=bar options is likely quite error prone and prevents someone
> from just typing "gcc hello.c && ./a.out". This will also only work if
> the source code one is compiling doesn't use these macros either, and
> there is no way to check at compile time.
> 
> So, for example, it wouldn't work for gcc bootstrap because
> lib1funcs.S and the c-runtime rely on the macros defined in
> xtensa-config.h.
> 
> So I'm wary of this approach.
> 
> Nonetheless, I would accept a patch that adds these -m options as long
> as they default to the values obtained from xtensa-config.h, and
> provides a reasonable method for the user to determine and pass all of
> the appropriate configuration values.
> 

OK, thank you
i'll start working on it.

-- 
Regards,
Oleksij

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