Hello list,
inside the automatic U-Boot patch tracking system a new ticket [DNX#2006033142000436] was created:
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On 10/6/05, Gregg Nemas gnemas@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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