On Fri, Nov 17, 2023 at 1:50 PM Miquel Raynal <miquel.ray...@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/15eb6abbefbf6916c28467b85485911dad3da6bc > Signed-off-by: Miquel Raynal <miquel.ray...@bootlin.com>
Yes, better not to write to reserved fields: Reviewed-by: Fabio Estevam <feste...@gmail.com>