[GSoC] BAT vs Pages on PowerPC [Please give a feedback]

[Hesham Moustafa]

On Sat, Jun 22, 2013 at 12:15 PM, Thomas Dörfler <
Thomas.Doerfler at embedded-brains.de> wrote:

> Hi,
>
> please note that some PPC derivatives (especially those for embedded
> projects) usually have eight DBATs and eight IBATs. This feature must be
> enabled (e.g. in a 603e core) with a special bit in one of the HID
> registers, but it will put the number of BAT ranges closer to what is
> needed in embedded computing.
>
> The advantage of BATs versus pages is definitively, that they usually
> get set up ONCE during startup and, due to their flexible size, will not
> need to be reloaded. Note that the pages usually only cover 4K of
> memory, and the "translation lookaside buffer" (TLB) only maintains the
> last 16-64 used pages, so there is always code involed reloading the
> required page info into the TLB. IMHO this reduces the realtime
> performance of the system.
>
> In short: Supporting both (BATs and pages) in libmm would be a good
> feature.
>
> Yes that's what I was thinking about. But how should I provide the user
with the option of choosing which method to use: BAT or Pages ?
Through a configuration option at build time? libmm should be general and
abstract in the way that it provides memory protection. Only a developer
who ports libmm should know about HW details like BAT and Pages.

> wkr,
>
> Thomas.
>
> On 22.06.2013 04:11, Chris Johns wrote:
> > Hesham Moustafa wrote:
> >> Eight: 4 for data and 4 for instructions. BAT areas can not overlap.
> >
> > Then I suggest we support for pages as well. There may be a need for
> > more areas in the future and I feel 4 may not be enough.
> >
> > Chris
> > _______________________________________________
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> > rtems-devel at rtems.org
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>
>
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