Re: [PATCH v2] Add io space MMAP for memory mapping devices and files

[Jose E. Marchesi]

> Hi again,
>
> Andreas Klinger <ak@it-klinger.de> schrieb am Sa, 06. Jan 20:42:
>> Hi Jose,
>> 
>> "Jose E. Marchesi" <jemarch@gnu.org> schrieb am Sa, 06. Jan 20:14:
>> > > I think that the approach of patch v2 could be optimized by a .set 
>> > > alignment
>> > > variable for those guys who are aware of such hardware constraints on 
>> > > their
>> > > architecture and want to optimize this. But I would set the default to a 
>> > > value
>> > > which works for all, which is the size of the address bus.
>> > 
>> > I think it is perfectly ok to have IOD specific configuration
>> > parameters, sure.
>> > 
>> > Let's make it a Poke variable that the user can customize, plus a
>> > setting in the registry so the .set dot-command works on it.  I suppose
>> > it shall have the name mmap in it, like pk_mmap_alignment.  WDYT?
>> 
>> great. It makes sense to name it mmap as it only applies to mmap io spaces.
> [...]
>
> After thinking about the problem again I came to a different solution which is
> much simpler, doesn't need an additional .set variable and avoids the signal
> handler as well as it is copying with the address bus size if possible, thus
> avoiding performance bottlenecks. The read function is as follows:
>
> static int
> ios_dev_mmap_pread (void *iod, void *buf, size_t count, ios_dev_off offset)
> {
>   struct ios_dev_mmap *dev_map = iod;
>   int align = sizeof(void*);
>   char *m = buf;
>
>   if (offset > dev_map->size || count > dev_map->size - offset)
>     return IOD_EOF;
>
>   /* copy unaligned bytes */
>   while (count && offset % align)
>     {
>       memcpy (m, dev_map->addr + offset, 1);
>       count--;
>       offset++;
>       m++;
>     }
>
>   /* copy with the address bus size */
>   while (count >= align)
>     {
>       memcpy (m, dev_map->addr + offset, align);

This is still copying a byte at a time, isn't it?

>       count -= align;
>       offset += align;
>       m += align;
>     }
>
>   /* copy remaining unaligned bytes */
>   while (count)
>     {
>       memcpy (m, dev_map->addr + offset, 1);
>       count--;
>       offset++;
>       m++;
>     }
>
>   return IOD_OK;
> }
>
> The write function is quite similar implemented in the same manner.
>
> WDYT?
>
> Andreas