[MPC52xx]Latency issue with DMA on FEC

[Jean-Michel Hautbois]

Hi lists !

I measured the latency and the jitter of the RX and TX ethernet paths
on my MPC5200 board.
The RX path is quite good, but the TX path can be slow.

[ 1218.976762] [mpc52xx_fec_start_xmit]Delay >30us for dma_map_single
=> 76364 ns
[ 1219.188405] [mpc52xx_fec_tx_interrupt]Delay >30us for
dma_unmap_single => 34515 ns
[ 1220.628785] [mpc52xx_fec_start_xmit]Delay >30us for
bcom_submit_next_buffer => 97273 ns
[ 1225.776784] [mpc52xx_fec_tx_interrupt]Delay >30us for
dma_unmap_single => 95273 ns

As one can see, this is obviously problematic.
The first function I analyzed is bcom_submit_next_buffer() => This
function doesn't do lots of things, except a call to mb().

I have been looking to the "MPC603e RISC Microprocessor User's Manual"
and especially the chapter named "2.3.4.7 Memory Synchronization
Instructions—UISA".

Here is a paragraph which explains a lot :

"The functions performed by the sync instruction normally take a
significant amount of time
to complete; as a result, frequent use of this instruction may
adversely affect performance.
In addition, the number of cycles required to complete a sync
instruction depends on system
parameters and on the processor's state when the instruction is issued."

I am using a real time kernel, and this is a problem, as it is not
deterministic to use this instruction.
Is there a way to avoid this ?

I will now focus on the dma_map_single() and dma_unmap_single functions...

Thanks in advance for your help,
Best Regards,

JM