On Feb 28, 2014 11:15 AM, "Mauro Carvalho Chehab" <m.chehab@xxxxxxxxxxx> wrote: > > Em Wed, 19 Feb 2014 17:39:07 -0800 > Andy Lutomirski <luto@xxxxxxxxxxxxxx> escreveu: > > > On Wed, Feb 19, 2014 at 11:03 AM, Luck, Tony <tony.luck@xxxxxxxxx> wrote: > > >> (I'm c/c Tony here, as he also shared the same concern that I had on a > > >> previous feedback about using I2C to talk with the DIMM). > > > > > > Correct - I've heard the same issues that reads on I2C can be misinterpreted > > > as writes ... and oops, you have a brick. > > > > Is this true on DDR3 DIMMs, i.e. anything that's compatible with > > LGA2011? If you plug a DIMM into an LGA2011 board's memory slot, > > then, one way or another, it's very likely that there will be TSOD > > traffic, if for no other purpose than to determine that there is no > > TSOD present. TSOD traffic consists of reads and writes, both with > > and without register numbers. (Sorry, I can never remember the smbus > > terminology here -- the relevant transactions are two-byte reads and > > two-byte writes, both with a command specified and without one. One > > of the bits in the iMC SMBUS registers tells the controller which kind > > of read to use to probe the thermometer.) > > An update on that: I double-checked with a DIMM manufacturer. > I was told that some DIMM models have write protect circuits but > others don't. > > So, yeah, accessing it could eventually brick the DIMM, depending > on the DIMM model, if such driver won't block I2C write ops or > if the BIOS is also trying to access the bus at the same time. I'm not sure I buy the argument about blocking writes -- as far as I know, i2c-i801 on desktop platforms has exactly the same issue. Or maybe I'm misunderstanding you. I'd be okay with adding code to block writes to SPD addresses, but I'm not sure I see the point. If root wants to brick a DIMM, I'm sure root can find a way. On the other hand, racing with BIOS is a real problem. I'd like to know how Intel plans to handle this. I suspect that POLL_EN is the right bit to use, but the docs are vague, and there could be strange BIOSes out there. (Grr. Everything would be saner if SMM didn't exist.) --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
