Hello,
A set of updates from our side.
0) We have started the process of migrating our repositories to new service. Please consider migrating the repository url on your side from git.linaro.org to https://git.codelinaro.org/linaro/qcomlt/kernel/ . The branch name states intact (release/db820c/qcomlt-5.15) The old repository location will mirror the new location, but it might take some time for the changes to propagate to it.
1) I have pushed a set of patches fixing two issues I narrowed down during my tests of the GPU. I will continue working on the driver.
2) Regarding fixing the GPU clocks. Please consider applying the attached patch, which should fix the minimum clock rate to 510 MHz.
If I interpret your last test results correctly, the issue really is in the GPU rate handling, so this patch should provide a workaround.