On Wednesday 08 November 2017 11:18 PM, Bjorn Helgaas wrote:
On Wed, Nov 08, 2017 at 02:15:05PM +0530, Vidya Sagar wrote:
On Wednesday 08 November 2017 04:20 AM, Bjorn Helgaas wrote:
On Tue, Oct 31, 2017 at 09:52:48AM +0530, Vidya Sagar wrote:
Programs T_cmrt (Commmon Mode Restore Time) and T_pwr_on (Power On)
values to get them reflected in ASPM-L1 Sub-States capability registers
Also adjusts internal counter values according to 19.2 MHz clk_m value
Signed-off-by: Vidya Sagar <vidyas@xxxxxxxxxx>
...
+u32 pcie_aspm_get_ltr_l_1_2_threshold(void)
+{
+ /* LTR L1.2 Threshold = 55us for all ports */
+ return ((0x37 << 16) | (0x02 << 29));
I know you've already worked through this, but let me think out loud
to try to figure this out myself.
ASPM defines Link power states L0, L0s, and L1. L1 PM Substates
extend that by adding L1.1 and L1.2. L1.2 presumably uses less power
and has a longer exit delay than L1.1 [sec 5.5].
Ports that support L1.2 must support Latency Tolerance Reporting (LTR)
[sec 6.18]. When LTR is enabled, a device periodically sends LTR
messages.
When ASPM puts a Link into L1, it chooses either L1.1 or L1.2 based on
LTR_L1.2_THRESHOLD and recent LTR messages. If there's no LTR
information it looks like LTR_L1.2_THRESHOLD doesn't matter and it
always chooses L1.2 [sec 5.5.1].
I don't see anything that writes PCI_EXP_DEVCTL2_LTR_EN, so I don't
think Linux ever enables LTR. Some BIOSes apparently enable it
(Google for "LTR enabled").
I think this needs to be done in aspm.c file. i.e. whenever
sub-system enables L1.2, it should enable
LTR_EN also.
That probably makes sense.
1) It seems like the LTR_L1.2_THRESHOLD value should be computed based
on the latency requirements of downstream devices. How did you
come up with 55us?
This is given by Tegra hardware folks.
I do not understand why this value should be dependent on the host
bridge. Can your hardware folks give more insight into this and how
they derived 55us?
I'm repeating myself, but the threshold (in combination with LTR
information) affects whether we enter L1.1 or L1.2. If I understand
correctly, this is all about the downstream devices and not at all
about the host bridge.
LTR_L1.2_THRESHOLD time is for the device to enter into and exit from
L1.2 state.
55us in case of Tegra is calculated based on the circuit design and
different latencies
involved in putting link through L1.2 entry-exit cycle.
Spec says in 5.5.4 that , quote "When programming LTR_L1.2_THRESHOLD
Value and Scale fields,
identical values must be programmed in both Ports" and my understanding
behind spec saying it explicitly
is that the end point should be made aware of what is the latency
requirement for L1.2 from root port,
just like the way end point makes root port aware of its L1.2 latency
requirement by sending an LTR message
upstream. By this, both root port and end point come to the same page
while taking a decision
to keep the link in L1.2 (or L1.1). Otherwise it may so happen that Tx
would be in L1.2 and Rx would be in L1.1.
Also, my understanding is that it is bound to be different on different
platforms as it comes from the way hardware
is designed.
...
3) We must support kernels with multiple host bridge drivers compiled
in, and the weak/strong symbol approach doesn't support using the
correct version, e.g., if we merge this patch, every system
containing the tegra driver would use this function, even if the
hardware had a different host bridge. Also, if another driver
implemented its own version, we'd have duplicate symbols.
Yes. Agree with this too.
How about using quirks framework for this?
If my assumption that "the threshold should be based on (a) the
latency requirements of downstream devices and (b) perhaps some global
power vs performance tradeoff" is correct, this doesn't really fit
into any kind of quirks or static computation, including the current
LTR_L1_2_THRESHOLD_BITS.
What happens if you keep all the Tegra-specific parts of this series,
i.e., you program the T_cmrt, T_pwr_on, and CLKREQ values, and enable
advertising of ASPM L1 capability, but leave out the
pcie_aspm_get_ltr_l_1_2_threshold() parts? (BTW, I think you should
reorder the series so you fix up all the delay values *before* you
advertise ASPM L1.)
I expect that to be functionally equivalent, but it would change the
L1.1 vs L1.2 tradeoff, so it might have some performance impact,
depending on what the downstream devices are.
Bjorn
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