[PATCH] tqma6: Fix DDR configuration

[Fabio Estevam]

On Fri, Nov 17, 2023 at 1:50 PM Miquel Raynal <miquel.raynal at 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.raynal at bootlin.com>

Yes, better not to write to reserved fields:

Reviewed-by: Fabio Estevam <festevam at gmail.com>