[DNX#2006033142000436] [U-Boot-Users] Re: Burst I/O on PPC440GP

[DENX Support System]

Hello list,

inside the automatic U-Boot patch tracking system a new ticket
[DNX#2006033142000436] was created:

<snip>
> On 10/6/05, Gregg Nemas <gnemas at gmail.com> wrote:
> > Stefan Roese <sr <at> denx.de> writes:
> >
> > > > I've been testing this by using the u-boot mw.l command. Do I need to use
> > > > PPC-specific instructions to perform burst I/O, or should an ordinary
> > > > programmed I/O (using 32-bit write operations) be automatically queued up
> > > > and converted to burst transactions?
> > >
> > > Do you want to transfer bigger chunks of data? If yes, you may want to take a
> > > look at the dma controller of the 440gp. It definately is able to generate
> > > burst transactions and probably the ideal solution for your needs.
> >
> > No, actually it will be relatively small transfers (16-128 bytes). I considered
> > using the DMA controller, but thought I would start out simple.
> >
> > > Another way to get burst transactions, is of course to use the cache, but I
> > > don't think that this is what you are looking for.
> >
> > You say "of course", but it really isn't at all clear to me how this works. Why
> > does enabling the cache allow burst operations? You are right that it is not
> > what I would normally want to do for device I/O memory.
> >
> > Thanks for your help.
> >
> > Gregg
> >
> >
> >
> >
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> >
> Gregg,
>  With caching disabled, the CPU considers that your external device
> needs to be updated with every memory write, since other devices may
> also access the memory and need to have the correct data.  With
> caching enabled, the CPU will store your writes internally until the
> memory cache is flushed, which forces the writes to the external
> device.  When flushing the cache (which is usually a relatively large
> amount of data, the cache will use burst transactions to write the
> data.  However, with caching enabled, there is the possibility that
> the data in the external device is not in sync with what the CPU
> thinks is there, so if other devices access the external device, they
> will get the incorrect data.
> 
> Chuck
> 
> 
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</snip>

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