On Fri, Nov 17, 2023 at 1:50 PM Miquel Raynal miquel.raynal@bootlin.com wrote:
Initially investigating a Linux network issue causing a lot of drop and poor network performances on a custom system based on a TQMA6A module (based on an iMX6Q), [1st link below].
I eventually correlated my observations with a contention at the NIC level when in concurrency with the graphics pipeline. Troubleshooting this in the kernel lead to disabling DMA bursts accesses made by the IPU in order to avoid triggering the QoS at the interconnect level, reducing from 50 to 10% the drop rate on eth0, [2nd link below]. The solution worked on my setup but not on others, which still suffered from abnormally high drop rates even with this "fix".
After looking a while into TQ Systems BSP I figured out a number of differences in recent U-Boot out-of-tree patches they had in their repository [3rd link]. Parsing the differences one after the other lead me to this final solution.
The reset pad of the DDR controller was apparently misconfigured, Bit 18-19 picturing the "DDR select field". The current value b11 is reserved. The only defined value as of version 6 of the iMX6Q manual was b00 "DDR3 and LPDDR2 mode". In practice no register difference has been spotted after changing this configuration but all issues tracked thus far just vanished. All previous fixes have been proven irrelevant. Just clearing this field solved all our network issues and the drop rate as measured by iperf3 felt back to 0%.
Link: https://lore.kernel.org/netdev/20231012193410.3d1812cf@xps-13/ Link: https://lists.freedesktop.org/archives/dri-devel/2023-October/428251.html Link: https://github.com/tq-systems/u-boot-tqmaxx/commit/15eb6abbefbf6916c28467b85... Signed-off-by: Miquel Raynal miquel.raynal@bootlin.com
Yes, better not to write to reserved fields:
Reviewed-by: Fabio Estevam festevam@gmail.com