Re: rfc: vmem(9) API/implementation changes

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Wed, 27 Jul 2011, David Young wrote:

> On Wed, Jul 27, 2011 at 04:58:23PM +0000, Eduardo Horvath wrote:
> > On Wed, 27 Jul 2011, David Young wrote:
> > 
> > > There are a couple of changes to the API that I would like to make.
> > > First, I don't think that vmem_addr_t 0 should be reserved for error
> > > indications (0 == VMEM_ADDR_NULL), but the API should change from
> > > this:
> > 
> > I'd recommend returning -1 on error.  0 is a valid address, but while -1 
> > is a valid address, when do you ever use this interface to allocate 
> > something that starts at address -1?  And it gets around all the noxious 
> > problems involved in returning data through reference parameters.
> 
> I don't know.  Suppose sizeof(vmem_addr_t) == sizeof(uint32_t).  Which
> of these cases should fail, and on which statement?
> 
> Case A:
> 
>      1        vm = vmem_create("test", 0xffffffff, 1, 0, NULL, NULL, NULL, 1,
>      2            VM_SLEEP, IPL_NONE);
>      3        p = vmem_alloc(vm, 1, VM_SLEEP);
> 
> Case B:
> 
>      1        vm = vmem_create("test", 0xfffffffe, 2, 0, NULL, NULL, NULL, 1,
>      2            VM_SLEEP, IPL_NONE);
>      3        p = vmem_alloc(vm, 2, VM_SLEEP);
> 
> Case C:
> 
>      1        vm = vmem_create("test", 0xfffffffe, 2, 0, NULL, NULL, NULL, 1,
>      2            VM_SLEEP, IPL_NONE);
>      3        p = vmem_alloc(vm, 1, VM_SLEEP);
>      4        q = vmem_alloc(vm, 1, VM_SLEEP);

All of them should fail in all the routines since you're specifying a 
quantum of 0.  This means you can only allocate multiples of 0 items from 
the list.

OTOH, how many times have you seen code like this:

foo(vmem_t *v) {
     void *p;

     vmem_alloc(v, 52, 0, (vmem_addr_t *)&p);

....


which has implementation defined functionality.

Eduardo