A lot has been said and spoken already about AMD's upcoming NAVI 12 (RX 5600) and 14 (RX 5500). More information from Linux drivers indicates that AMD Navi 12 gets a 256-bit memory bus and the RX 5600 128-bit, likely GDDR6.

The news arrives today though a user at Germany based 3DCenter forums called Berniyh, he found Navi 12 and Navi 14 in Linux drivers, the two GPUs could end up in the RX 5800 and RX 5600 cards respectively. Navi 12 is mentioned to get a 256-bit memory bus. Navi 14 would, according to previously surfaced drivers, get versions with 3, 4 and 8 GB volume graphics memory but on a 128-bit memory bus. 

»···» ··· case CHIP_NAVI10:
»···» ··· case CHIP_NAVI12:
»···» ··· »··· pc_lines = 1024;
»···» ··· »··· break;
»···» ··· case CHIP_NAVI14:
»···» ··· »··· pc_lines = 512;
»···» ··· »··· break;
"···"···default:
»···» ··· »··· assert (0);
"···" ···}

"···" ··· if (info-> chip_class> = GFX10) {
»···» ··· »··· info-> pbb_max_alloc_count = pc_lines / 3;
»···» ···} else {
»···» ··· »··· info-> pbb_max_alloc_count =
»···» ··· »···» ··· MIN2 (128, pc_lines / (4 * info-> max_se));
"···" ···}
"···}

»··· if (info-> chip_class> = GFX10) {
»···» ··· switch (info-> family) {
»···» ··· case CHIP_NAVI10:
»···» ··· case CHIP_NAVI12:
»···» ··· »··· info-> num_sdp_interfaces = 16;
»···» ··· »··· break;
»···» ··· case CHIP_NAVI14:
»···» ··· »··· info-> num_sdp_interfaces = 8;
»···» ··· »··· break;
"···"···default:
»···» ··· »··· assert (0);
"···" ···}
"···}

For the 5700 (Navi 10) A GPU with 8GB VRAM has a 256-bit wide memory bus, and is referred to as 'num_sdp_interfaces = 16'. Since NAVI12 is listed at 16 as well that would be 256-bit, NAVI14 at 8 SDP this is 128-bit. Interesting is the mention PC_lines, you'd think that's the number of shader procs, but really that cannot be the case. It is expected that the new cards will see the light of day in October.

AMD Navi 12 Gets 256-bit memory bus according to Linux drivers, Radeon RX 5600 128-bit