On Fri, Mar 24, 2017 at 2:15 AM, Berj K Chilingirian berjc@mit.edu wrote:
Hi,
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:
Patch against the source is easier to diagnose for possible mistakes than verbal expression of what you believe you are doing. Eg. the address you write to makes a difference.
Kyösti