This discussion has been locked.

You can no longer post new replies to this discussion. If you have a question you can start a new discussion

AHB Bufferable/Non-bufferable write

Note: This was originally posted on 12th September 2008 at http://forums.arm.com

Hi,

Please clarify the following issue related to AHB write:

If HPROT[2] = 1, AHB write is bufferable and we need to provide OKAY response as soon as the AHB slave interface receives data without caring that the data is actually written into the memory or not.

if HPROT[2] = 0, AHB write is non-bufferable, AHB slave interface needs to provide response to the master only when the data is actually written into the memory.

The issue is that if we see this non-bufferable transaction, the state of HRESP will by default be OKAY (HREADY is HIGH to accept the next data) and it means that we are providing the response in advance for non-bufferable write.

Please clarify how to distinguish between the two transactions.
If possible, please provide waveforms for INCR undefined length & non-bufferable write

Parents

0 Colin Campbell over 11 years ago

Note: This was originally posted on 16th September 2008 at http://forums.arm.com

Hi Hari,

I would think there are problems with both approaches.

For approach 1, where you only provide a real response for the last transfer in a burst, this will improve efficiency, but it relies on the master being designed to interpret this final response as applying to the complete burst. Just something your software needs to be aware of when handling the ERROR.

Also, you would need to make sure that this final real response you return is a summation of all the real responses for the burst, so if only the 3rd transfer in an INCR8 burst received an ERROR, you would need to return ERROR for transfer 8, even if it actually was an OKAY from the slave.

I am only looking at ERROR responses because for SPLIT and RETRY you would need to locally handle them in your buffering logic, while adding wait states to the master to stall it.

For approach 2, what will you do if a master attempts an INCR burst ?

If you do not support them, would you signal ERROR (which would be a problem for most masters) ?

Sorry, I know I am only raising problems here and not solutions, but if you want to fully support bufferable writes, you should only do so if you know absolutely that the transfers will complete with OKAY responses, or if you don't care what the response is from the slave.

Your approach 1 is probably the best compromise between efficiency and complexity, assuming the master knows how to interpret the final response, as it does allow buffered writes while still returning a real response.

Approach 2 I would think is too restrictive.

I guess it really depends on your application, you know what types of bursts your master(s) will attempt, what responses are possible from your slave(s), and what sort of performance you need, so it looks like you are considering all the right sorts of things.

Good luck.

JD
Cancel
Up 0 Down

Cancel

Reply

0 Colin Campbell over 11 years ago

Note: This was originally posted on 16th September 2008 at http://forums.arm.com

Hi Hari,

I would think there are problems with both approaches.

For approach 1, where you only provide a real response for the last transfer in a burst, this will improve efficiency, but it relies on the master being designed to interpret this final response as applying to the complete burst. Just something your software needs to be aware of when handling the ERROR.

Also, you would need to make sure that this final real response you return is a summation of all the real responses for the burst, so if only the 3rd transfer in an INCR8 burst received an ERROR, you would need to return ERROR for transfer 8, even if it actually was an OKAY from the slave.

I am only looking at ERROR responses because for SPLIT and RETRY you would need to locally handle them in your buffering logic, while adding wait states to the master to stall it.

For approach 2, what will you do if a master attempts an INCR burst ?

If you do not support them, would you signal ERROR (which would be a problem for most masters) ?

Sorry, I know I am only raising problems here and not solutions, but if you want to fully support bufferable writes, you should only do so if you know absolutely that the transfers will complete with OKAY responses, or if you don't care what the response is from the slave.

Your approach 1 is probably the best compromise between efficiency and complexity, assuming the master knows how to interpret the final response, as it does allow buffered writes while still returning a real response.

Approach 2 I would think is too restrictive.

I guess it really depends on your application, you know what types of bursts your master(s) will attempt, what responses are possible from your slave(s), and what sort of performance you need, so it looks like you are considering all the right sorts of things.

Good luck.

JD
Cancel
Up 0 Down

Cancel

Children

No data