[coreboot] Question about PCIe separate reference clock

[Predrag Vidic]

Hi Zheng,

Schematic is simple and OK. I assume you are trying to run Wolf
VPXxx-E8860-xxxx or similar GPU borad. You can't do much there if there is
a hardware problem but it is most possibly a software setup problem.

VPX does not have incomming clock so it is by default in async_clock mode.
The question is did you allowed that mode on the main board side?

Asynchronous clock mode is not default mode for a motherboard. Check if the
mode is allowed. Second, check if the other PCIe device can work on the
motherboard PCIe slot.

Regards,
Predrag


On Fri, Jan 13, 2017 at 6:44 AM, Zheng Bao <fishbaoz at hotmail.com> wrote:

> Here is the ref clk part. Please review.
>
> No refclk routes to VPX connector.
>
>
> Thanks.
>
>
> Zheng
> ------------------------------
> *From:* Predrag Vidic <pvidic at gmail.com>
> *Sent:* Thursday, January 12, 2017 6:53 PM
> *To:* Zheng Bao
> *Cc:* Zoran Stojsavljevic; coreboot at coreboot.org
>
> *Subject:* Re: [coreboot] Question about PCIe separate reference clock
>
> Hi Zheng,
>
> Without knowing the particular solution for the PCIe transmitter you have
> on your board, I'd check refclk pins on your design for the proper
> termination. The problem can be in irregular readings from the pins.
>
> Reragds,
> Predrag
>
> On Thu, Jan 12, 2017 at 3:26 PM, Zheng Bao <fishbaoz at hotmail.com> wrote:
>
>> Q: do you use local xtal attached to   Si52111-B5 to generate local PCIe
>> 25MHz clock?
>>
>> Zheng: yes
>>
>>
>> Q: If you dothis, my next question is how you synchronize these two
>> clocks: Local
>> PCIe 25 MHz and common reference clock from CPU?
>>
>> Zheng: We do NOT sync these two.
>>
>>
>> Q: Since these two clocks, as I understand above scenario, are
>> asynchronous to each other?!
>>
>> Zheng: yes. Asynchronous.
>>
>>
>> The VPX connector does not have a PCIe ref clock signal, so we can not
>> send CPU PCIe ref clock to
>>
>> device.  The PCIe spec says if the separate refclk on devices should be
>> 100MHz ± 300PPM, with SSC
>>
>> off.  We believe our board meet this requirement. So we doubt the problem
>> lies in PCI configration space.
>>
>>
>> Zheng
>>
>>
>>
>> ------------------------------
>> *From:* Zoran Stojsavljevic <zoran.stojsavljevic at gmail.com>
>> *Sent:* Thursday, January 12, 2017 9:22 AM
>> *To:* Zheng Bao; Predrag Vidic
>> *Cc:* coreboot at coreboot.org
>> *Subject:* Re: [coreboot] Question about PCIe separate reference clock
>>
>> Hello Zheng,
>>
>> For decades, I've been FW/SW engineer, but I do understand a little
>> bit of a HW. I have looked into the Si52111-B5 data sheet for
>> clarification.
>>
>> My problem here is to understand, your use case: do you use local xtal
>> attached to   Si52111-B5 to generate local PCIe 25MHz clock? If you do
>> this, my next question is how you synchronize these two clocks: Local
>> PCIe 25 MHz and common reference clock from CPU?
>>
>> Since these two clocks, as I understand above scenario, are
>> asynchronous to each other?!
>>
>> Please, clarify for us your use case.
>>
>> Thank you,
>> Zoran
>> _______
>>
>> On 1/12/17, Zheng Bao <fishbaoz at hotmail.com> wrote:
>> > Our VPX design uses separate reference clock source, which is
>> Si52111-B5 (No
>> > spread), instead of common ref clock from CPU.
>> > Now The system is unstable. Reading PCIE configuration space is unstable
>> > too. (If we add some fly wire to make it work with common ref clock, the
>> > system becomes stable.)
>> >
>> > (abstracted from PCIe spec: 12 Slot Clock Configuration - This bit
>> indicates
>> > that the
>> > component uses the same physical reference clock that the
>> > platform provides on the connector. If the device uses an
>> > independent clock irrespective of the presence of a reference
>> > clock on the connector, this bit must be clear.
>> > For a multi-Function device, each Function must report the
>> > same value for this bit.)
>> >
>> > Based on my understanding, the BIOS need to read bit "Slot Clock
>> > Configurationclear" to see if
>> > separate ref clock is used.  BIOS then write bit "Common Clock
>> > Configuration".
>> >
>> > On our board, the bit "Slot Clock Configuration" is always 1, which I
>> assume
>> > should be 0.
>> >
>> > My question is, how the hardware affect the bit "Slot Clock
>> Configuration"?
>> > How do we need to design our board to make the bit "Slot Clock
>> > Configuration" be 0?
>> >
>> > Thanks.
>> >
>> > Zheng
>> >
>> >
>>
>
>
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