[coreboot] Disabling Auto-Refresh of DRAM Using Coreboot

Berj K Chilingirian berjc at mit.edu
Fri Mar 24 01:15:48 CET 2017


I was hoping to get some advice/guidance on how to disable the automatic refresh of the DRAM during the ROM stage of coreboot.

I am booting an ASUS F2A85-M motherboard with a AMD A6-5400k processor (Trinity core) using coreboot. The processor is initialized via the AGESA bootstrap protocol.

I would like to use the AGESA software<https://github.com/coreboot/coreboot/tree/master/src/vendorcode/amd/agesa/f15tn> in order to measure the retention time of DRAM cells (i.e. the time a DRAM cell holds its value with refresh shut off). To do this, I must (1) run my code in cache-as-RAM mode because DRAM becomes unreliable without refresh and (2) have a way to switch DRAM refresh on/off from software. (1) is accomplished by running my code in the cache-as-RAM phase of coreboot (i.e. the ROM stage). I am, however, unclear on how to accomplish (2) and I was hoping to receive some pointers/guidance on how to do this.

The AGESA protocol seems to handle memory initialization in the AmdInitPost<https://github.com/coreboot/coreboot/blob/master/src/vendorcode/amd/agesa/f15tn/Proc/Common/AmdInitPost.c#L223> phase, specifically in AmdMemAuto<https://github.com/coreboot/coreboot/blob/master/src/vendorcode/amd/agesa/f15tn/Proc/Mem/Main/mmflow.c#L135>  In reading this code I discovered a call with which one could disable/enable the auto-refresh feature of the DRAM. Specifically,

MemNSetBitFieldNb<https://github.com/coreboot/coreboot/blob/master/src/vendorcode/amd/agesa/f15tn/Proc/Mem/NB/mnreg.c#L218> (NBPtr, BFDisAutoRefresh, 1);

where NBPtr is a pointer to a data structure representing the northbridge configuration and BFDisAutoRefresh<https://github.com/coreboot/coreboot/blob/master/src/vendorcode/amd/agesa/f15tn/Proc/Mem/mm.h#L365> is a bit field which can be set to 1 in order to disable auto-refresh of the DRAM (see AMD BKDG<http://support.amd.com/TechDocs/42300_15h_Mod_10h-1Fh_BKDG.pdf>, page 344 on DisAutoRefresh).

With this interface, I thought I could (1) write a value to memory (using movnti instruction to bypass the cache), (2) disable auto-refresh using MemNSetBitFieldNb, (3) wait some number of seconds, (4) enable auto-refresh using MemNSetBitFieldNb, and (5) check whether the value has persisted over the given period of time. Unfortunately, I discovered that even after waiting 3 minutes, the value in DRAM persisted (contrary to what seems to be expected by the literature<http://www.pdl.cmu.edu/PDL-FTP/NVM/dram-retention_isca13.pdf>). Note that I perform this procedure inside the AmdMemAuto<https://github.com/coreboot/coreboot/blob/master/src/vendorcode/amd/agesa/f15tn/Proc/Mem/Main/mmflow.c#L285> procedure of the AGESA code once all memory initialization tasks have completed and before teardown occurs.

Worried that the processor may be caching the value, I decided to flush data at the address before disabling auto-refresh with the following interface provided by the AGESA codebase:

MemUFlushPattern<https://github.com/coreboot/coreboot/blob/master/src/vendorcode/amd/agesa/f15tn/Proc/Mem/Main/mu.c#L219> (Addr, 1);

where 1 simply represents the number of cache lines to flush from the address Addr. This, however, proved not to solve my issue.

I am curious if anyone has dug into the AGESA vendor code and has any experience advice for how to approach the functionality I am trying to achieve.

Thank you for all your help!

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