Re: [RFC] Platform information services

[Javier Martín]

El vie, 15-08-2008 a las 20:14 +0300, Vesa Jääskeläinen escribió:
> Javier Martín wrote:
> > El vie, 15-08-2008 a las 19:31 +0300, Vesa Jääskeläinen escribió:
> >> Javier Martín wrote:
> >>> WRT "kernel and modules going hand by hand", think about external
> >>> modules: if the drivemap module is finally rejected for introduction in
> >>> GRUB, I will not scrap it, but keep it as a module external to the
> >>> official GNU sources and possibly offer it in a web in the form of
> >>> patches to the official GRUB2. In this case, changes made to the kernel
> >>> would not take into account that module, which would break if I weren't
> >>> monitoring this list daily.
> >> Then it is really your problem ;)
> > Indeed, but bitrot is not just the real of external modules: it's
> > happening right now even within the GRUB trunk as you admit in the
> > "Build problems on powerpc" thread...
> 
> And? If Power PC maintainer is nowhere to update to newest additions
> then it is rightly in in-compilable state and if it rots too long its
> support will be removed. That's life.
Yes, but you implicitly and particularly Robert explicitly argued that
when kernel changes devs here take the time to update modules in
consonance and vice versa.  The PPC build problem shows that, for
whatever reason, this is not always true.
> 
> >>> Additionally, the cost of this function in platforms which don't have
> >>> any structs registered yet, as the function could be a stub like this:
> >>>
> >>> void* grub_machine_get_platform_structure (int stidx)
> >>> {
> >>>   grub_error (GRUB_ERR_BAD_ARGUMENT, "Struct %d not supported", stidx);
> >>>   return 0;
> >>> }
> >>>
> >>> The kernel space taken would most likely be less than 50 bytes. For
> >>> i386-pc, it could be like this (also lightweight) function:
> >>>
> >>> void* grub_machine_get_platform_structure (int stidx)
> >>> {
> >>>   grub_errno = GRUB_ERR_NONE;
> >>>
> >>>   switch (stidx)
> >>>   {
> >>>   case GRUB_MACHINE_I386_IVT:
> >>>     return /* Call to asm function that runs SIDT in real mode */ ;
> >>>   case GRUB_MACHINE_I386_BDA:
> >>>     return (void*)0x400;
> >>>   default:
> >>>     grub_error (GRUB_ERR_BAD_ARGUMENT, "Struct %d not supported",
> >>>                 stidx);
> >>>     return 0;
> >>>   }
> >>> }
> >> And what lets assume couple of extra platforms... how about
> >> x86-32bit-efi and ppc. What do they do?
> >>
> >> Implement their own enum entries for those indexes and only use their
> >> own indices...?
> > At first, they would just have the stub which does not recognize any
> > index, but yes, i386-efi devs could decide that certain
> > firmware-provided structure (like a video modes info table or such, I
> > don't know the internals of EFI) might be interesting to a module
> > they're creating, so they create an index for it and add it to the
> > version of the function in their platform.
> > 
> > If I had not mentioned it before, the function would be declared in a
> > cross-platform file, but _implemented_ in platform-specific files, and
> > the indices would be declared in the platform-specific machine.h.  Thus,
> > there would not be a "single" indices namespace: structure #1 might be
> > the IVT in i386-pc, but some devices info table in powerpc-ieee1275.
> > 
> >> Where here we are sharing any code? (if we do not count the name of the
> >> fuction.) Interface is kinda useless if there is no possibility that
> >> no-one is sharing its functionality...
> > The idea is a single function to retrieve the addresses of
> > firmware-provided/used structures.  This includes the IVT and BDA in
> > i386-pc, but as I said before it could also be used by other platforms
> > for their own structures.  The alternative would be just creating such
> > "get struct X" functions on each platform as they are needed, but I
> > imagined that a single interface (with such a low cost in space) would
> > be a more elegant solution.
> 
> I still do not see the need to create cross platform function that
> cannot be used for cross platform purposes. It is much more reasonable
> to write such needs as in kernel for the platform or in platform
> specific modules.
> 
> I do not have a problem with function to retrieve pointer to some
> platform specific function on platfrom specific code. That is normal
> life of the platform.
Well, think of this as a platform-specific function to retrieve the
address of platform-specific (firmware-provided) structures. The only
difference is that in my proposal such a function has the same signature
(prototype) for every platform.
> 
> And if we think about code safety, casting is bad. And if you keep
> indices colliding on different platforms then you are just calling for
> problems. It completely removes any advantage what this kinda wrapper
> could have had.
Why? any module that is "advanced" (in the sense of complicated) enough
to require the use of platform-specific structures will be
platform-specific itself, or at least the part of it that accesses the
structures, and thus the risk of indices actually colliding is nearly
zero.

-Habbit
> 
> 
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