glxinfo shows GL_ARB_texture_compression but not GL_S3_s3tc nor GL_EXT_texture_compression_s3tc.
bool enable_s3tc = rscreen->info.drm_minor >= 31;
(Not sure how I can see my driver's version.)
I think this refers to KMS_DRIVER_MINOR in the kernel drivers/gpu/drm/radeon/radeon_drv.c.
The original patch "drm/radeon: add si tile mode array query" increased it to 31.
So it looks like it will not be supported before 3.10.
(In the actual merging to agd5f/drm-next-3.10 KMS_DRIVER_MINOR was increased to 33, so I guess the detection will be slightly wrong)
But it does seem to work with 3.9 when I simply set it to true. xonotic would complain that texture compression is not supported without it, but with it it doesn't complain and renders completely fine. Does it not work in every case?