Linux/DRA756: How to config the 2GB DDR on DRA75x

Hi Joe

The 16M region configured in DMM section-3 is reserved region for Tiler to trap unmapped tiler entries. The u-boot deducts this 16MB area from total DDR size 2G before passing to kernel. The DDR size is 2GB minus 16MB.

Regards
Ravi

Joe
In general the Tiler region is used for supporting Scatter-gather functionality (in tiler 1-D mode). By default the video buffers(used for display, multimedia) are allocated as scatter gather list of pages and mapped contiguously using the Tiler entries. If you do not want to use Tiler at all, we will need other means to manage allocation of contiguous large buffers needed for video buffers - linux supports CMA region as an option - though this may not be the default path for video buffer allocation.

Also,Please clarify on the SDK version that you are using

If Tiler is used(which is the default option), then we need to able to trap access to unmapped entries in the Tiler table.

Referring back to the GIt commit message that added support for the tiler trap section
" The unmapped entries in tiler space are set with
values 0xFF. So creating a DMM section of
size 16MB at 0xFF000000 with ADDRSPACE set to 0x2.

This way all the unmapped entry accesses to tiler
will be trapped by the EMIF and a error response
is sent to the L3 interconnect. L3 errors are
inturn reported to MPU.

Note that here the tiler trap section is overlapping
with the actual ddr physical space and we lose 16MB
out of the total 2GB. "


In summary, would anticipate that the Tiler module gets used for mapping scatter gather page list for video buffers and the 16MB region is required in such cases to trap unmapped entry accesses to tiler.

Regards
Sriram