cvs commit: src/sys/dev/bce if_bcereg.h

[M. Warner Losh]

In message: <20060427045314.C1064 at epsplex.bde.org>
            Bruce Evans <bde at zeta.org.au> writes:
: On Wed, 26 Apr 2006, M. Warner Losh wrote:
: 
: > In message: <444F0923.8050508 at samsco.org>
: >            Scott Long <scottl at samsco.org> writes:
: > : Matthew Jacob wrote:
: > : >
: > : >> Supporting sizes >= 4G sounds unreasonable.  How can a single device
: > : >> need or even address so much space, even on 64-bit arches?  For vm,
: > : >> virtual memory is sort of a device, but even it is limited to 4G on
: > : >> 32-bit arches, and PAE on i386 isn't pessimized by using a larger than
: > : >> necessary vm_size_t.
: > : >
: > : > I have need to support and help people sell machines that use 32GB of
: > : > directly addressable memory. In fact, the EM64T cheat will shortly
: 
: Such machines cannot be i386's.
: 
: > : > become an embarrasment to Intel when people find out that EM64T with PAE
: > : > is *not* the same as Opteron (36 vs. 40).
: > : >
: > : > I'm afraid I don't understand the 'unreasonable' argument here. Linux is
: > : > eating your lunch today. Do you want it to eat your dessert as well?
: 
: > : bus_size_t is used for things like measuring transfer segment size.
: > : There is little chance that Linux, Windows, FreeBSD, or any other OS
: > : is ever going to try to DMA more than 2^32 bytes of data in a single
: > : bus transaction.  Maybe you could contrive a silly infiniband device
: > : to do it.  Anyways, it has no bearing on whether the CPU, memory
: > : controller, or PCI buses can do 64 bit addressing.
: >
: > bus_size_t is for differences between two bus_addr_t quantities, since
: > it specifies the size of resources on a bus.  It is also used for
: > transfer sizes and the like.  That's why I think it should be a 64-bit
: > quantity: 64-bit - 64-bit = 64-bit.
: 
: No it isn't (or shouldn't be).  It should be for the size of individual
: "objects", like size_t and vm_size_t are for the size of individual
: objects.  In C, size_t is sometimes abused for differences, but shouldn't
: be, and ptrdiff_t is specifically for differences; however, prtdiff_t
: is only required to work for differences within objects, and since it
: needs an extra bit for the sign but rarely has one, it rarely actually
: works for external differences unless the address space is constrained.
: In vm, addresses are confused with differences to the point where
: vm_addr_t doesn't exist and the basic type vm_offset_t is used for
: addresses.  I think vm rarely needs offsets or differences except for
: the special case of offsets from address 0, so this is mostly a spelling
: error.

In the PCI 64-bit address space, objects can be larger than 32-bits in
the 2 DWORD bars.  To support these large allocations, bus_space_t
needs to be 64-bit.  While few (if any) devices today have such large
allocations, it is allowed in the 2.2 PCI standard (and subsequent
revisions).

Warner