[Cbe-oss-dev] [PATCH] spufs: avoid using backing operation for wbox write

[Andre Detsch]

On Sunday 05 August 2007 16:18:07 Arnd Bergmann wrote:
> But the SPU_WrEventAck word _is_ written at the context restore time,
> so we must have a way to give it a specific value. AFAICS, all that
> needs to be done is to change the save channel data in
> spu_state->chnldata[2] to restore the new correct value to
> SPU_WrEventAck through the channel access facility.

Channel 2 is not saved/restored. Not in the code, not in the documentation.
Section 12.5 of CBE_Handbook_v1.1_24APR2007_pub.pdf details the context switch 
process, and it does not mention channel 2. For example, item 39 refers the 
events channels, but does not include channel SPU_WrEventAck (channel 2)

"39. For the following channels:
• SPU_RdEventStat (channel index 0)
• SPU_WrEventMask (channel index 1)
• SPU_RdSigNotify1 (channel index 3)
• SPU_RdSigNotify2 (channel index 4)
• MFC_RdTagStat (channel index 24)
• MFC_RdListStallStat (channel index 25)
• MFC_RdAtomicStat (channel index 27)"

Latter, like on item 45, additional channels are referenced, but nothing about 
channel 2.

However, saving/restoring channel 2 was my first try (hoping that the 
documentation could be incomplete). Attached is a patch with such a try. The 
results are the same for the simulator and blade, but on simulator, we can 
observe the channel values more easily. The test case I have hangs after the 
following messages from the kernel:

Saving Channel 2 0x4D0 (spu=7)
Saving Channel 2 0x4D0 (spu=6)
Setting Channel 2 to 0x4C0 (spu=7)
Setting Channel 2 to 0x4C0 (spu=6)

Looking at the channels debug window, both SPU 6 and 7 still have Channel 2 == 
0x4D0 (although I'm explicitly trying to write 0x4C0, which correspond to the 
same mask, only without the MB event bit unset). I've also tried a different 
write order (writting channel 2 at the end of the function, after the other 
channels), and also, forcing SPU_WrEventMask (channel 1) to have a non-zero 
value, but nothing has helped.

PS: like channel 1, channel 2 does not have an associated count. Simulator 
even complains if one tries to write on channel 2 count (e.g., "WARNING: 
92790504834: SPE7: Attempt to write channel count for CH 2 with no associated 
count is ignored"). 

--
Andre Detsch
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