Re: [COMMITTED PATCH] Move tilegx, tilepro, and linux-generic from ports to libc.

[Chris Metcalf <cmetcalf at ezchip dot com>]

On 03/19/2015 12:44 PM, H.J. Lu wrote:
> On Thu, Mar 19, 2015 at 9:31 AM, Chris Metcalf <cmetcalf@ezchip.com> wrote:
> > On 03/18/2015 07:59 AM, H.J. Lu wrote:
> > > Any particular reason to add sysdeps/unix/sysv/linux/generic/bits/statfs.h
> > > when there is sysdeps/unix/sysv/linux/bits/statfs.h already? Why can't it be
> > > used?
> >
> > The distinction lies with the the fields defined by __field64 in the
> > "generic" version.  For the case without __USE_FILE_OFFSET64, but when ILP32
> > mode is involved, the base linux version uses plain __fsblkcnt_t, i.e.
> > unsigned long, so just 32 bits.  However, it needs to be a 64-bit field,
> > where either the first or second 32-bit subfield is padding depending on
> > endianness, and the field value is the other 32-bit subfield.  We would need
> > to adopt the __field64 scheme for the base version in some way that allowed
> > platforms to turn it on as needed (depending on whether statfs is filled
> > with a statfs64 syscall or a plain 32-bit statfs syscall on older, non
> > asm-generic 32-bit kernels).  This is doable but fiddly to get exactly
> > right.
> >
> > Another difference I see is easy to work around.  The "generic" linux
> > version uses __SWORD_TYPE where the base version uses __fsword_t (changed by
> > hjl in 2012).  If the generic platforms (tile, aarch64, nios2) switched,
> > this would be OK since the types are the same on those platforms.  (Only
> > alpha and x32 define them differently.)
> There are
>
> sysdeps/unix/sysv/linux/mips/bits/statfs.h
> sysdeps/unix/sysv/linux/alpha/bits/statfs.h
> sysdeps/unix/sysv/linux/bits/statfs.h
> sysdeps/unix/sysv/linux/s390/bits/statfs.h
>
> If sysdeps/unix/sysv/linux/bits/statfs.h can't be used for
> a Linux platform, it should define its own statfs.h, like
> alpha, mips and s390.

The linux/generic layer is essentially an intermediate  layer for all 
new Linux ports that use the asm-generic syscall interface. Rather than 
"inheriting" directly from the sysdeps/unix/sysv/linux/, newer ports 
inherit from sysdeps/unix/sysv/linux/generic/, which tweaks whatever 
needs tweaking to support the fact that a far smaller set of syscalls is 
available on such platforms.  This means tile, aarch64, and nios2 don't 
have to each provide their own replacement statfs.h.

> Having both
>
> sysdeps/unix/sysv/linux/generic/bits/statfs.h
>
> and
>
> sysdeps/unix/sysv/linux/bits/statfs.h
>
> is very strange.

Yes, I don't love the naming either, but no one could come up with 
anything better when this was initially introduced.

It may be the case that we can do some unification along the lines I 
suggested in my previous email, but the tricky part is figuring out 
programmatically which platforms are using the new API (and thus don't 
have a version of statfs with 32-bit fsword_t) and which are using the 
old API.  Or, moving forward, which platforms are using the LP64 version 
of the syscall even in ILP32 mode (x32, aarch64:ILP32).  Since this is a 
public header we don't have kernel-features.h.  I'm not sure where else 
we would normally place this kind of information in the headers.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com