The PHY register block is 64KB, so with a register stride of 4 the last accessible register sits at offset 0xfffc. max_register names 0x10000, one register past the end of the mapping: dumping the registers through the regmap debugfs interface reads beyond the ioremapped region and oopses on the unmapped page. The oops fires with the regmap lock held, so later PHY operations deadlock.