[RESEND PATCH v2] PCI: mediatek: Delay 100ms to wait power and clock to become stable

[qizhong.cheng]

On Wed, 2021-12-08 at 11:18 +0100, Pali Rohár wrote:
> On Wednesday 08 December 2021 14:07:57 qizhong.cheng wrote:
> > On Tue, 2021-12-07 at 22:12 -0600, Bjorn Helgaas wrote:
> > > On Tue, Dec 07, 2021 at 10:00:43PM +0100, Mark Kettenis wrote:
> > > > > Date: Tue, 7 Dec 2021 11:54:16 -0600
> > > > > From: Bjorn Helgaas <helgaas at kernel.org>
> > > > > 
> > > > > [+cc Marc, Alyssa, Mark, Luca for reset timing questions]
> > > > 
> > > > Hi Bjorn,
> > > > 
> > > > > On Tue, Dec 07, 2021 at 04:41:53PM +0800, qizhong cheng
> > > > > wrote:
> > > > > > Described in PCIe CEM specification sections 2.2 (PERST#
> > > > > > Signal) and
> > > > > > 2.2.1 (Initial Power-Up (G3 to S0)). The deassertion of
> > > > > > PERST#
> > > > > > should
> > > > > > be delayed 100ms (TPVPERL) for the power and clock to
> > > > > > become
> > > > > > stable.
> > > > > > 
> > > > > > Signed-off-by: qizhong cheng <qizhong.cheng at mediatek.com>
> > > > > > Acked-by: Pali Rohár <pali at kernel.org>
> > > > > 
> > > > > ...
> > 
> > 1)
> > 2)
> > Thanks for your reminding and suggestion.
> > 
> > > > > 3) Most importantly, this needs to be reconciled with the
> > > > > similar
> > > > > change to the apple driver:
> > > > > 
> > > > >   
> > > > > https://lore.kernel.org/r/20211123180636.80558-2-maz@kernel.org
> > > > > 
> > > > > In the apple driver, we're doing:
> > > > > 
> > > > >   - Assert PERST#
> > > > >   - Set up REFCLK
> > > > >   - Sleep 100us (T_perst-clk, CEM r5 2.2, 2.9.2)
> > > > >   - Deassert PERST#
> > > > >   - Sleep 100ms (not sure there's a name? PCIe r5 6.6.1)
> > > > > 
> > > > > But here in mediatek, we're doing:
> > > > > 
> > > > >   - Assert PERST#
> > > > >   - Sleep 100ms (T_pvperl, CEM r5 2.2, 2.2.1, 2.9.2)
> > > > >   - Deassert PERST#
> > > > > 
> > > > > My questions:
> > > > 
> > > > My understanding of the the Apple PCIe hardware is somewhat
> > > > limited
> > > > but:
> > > > 
> > > > >   - Where does apple enforce T_pvperl?  I can't tell where
> > > > > power
> > > > > to
> > > > >     the slot is turned on.
> > > > 
> > > > So far all available machines only have PCIe devices that are
> > > > soldered
> > > > onto the motherboard, so there are no "real" slots.  As far as
> > > > we
> > > > can
> > > > tell the PCIe power domain is already powered on at the point
> > > > where
> > > > the m1n1 bootloader takes control.  There is a GPIO that
> > > > controls
> > > > power to some devices (WiFi, SDHC on the M1 Pro/Max laptops)
> > > > and
> > > > those
> > > > devices are initially powered off.  The Linux driver doesn't
> > > > currently
> > > > attempt to power these devices on, but U-Boot will power them
> > > > on if
> > > > the appropriate GPIO is defined in the device tree.  The way
> > > > this
> > > > is
> > > > specified in the device tree is still under discussion.
> > > 
> > > Does this mean we basically assume that m1n1 and early Linux boot
> > > takes at least the 100ms T_pvperl required by CEM sec 2.2, but we
> > > take
> > > pains to delay the 100us T_perst-clk?  That seems a little weird,
> > > but
> > > I guess it is clear that REFCLK is *not* enabled before we enable
> > > it,
> > > so we do need at least the 100us there.
> > > 
> > > It also niggles at me a little that the spec says T_pvperl starts
> > > from
> > > *power stable* (not from power enable) and T_perst-clk starts
> > > from
> > > *REFCLK stable* (not REFCLK enable).  Since we don't know the
> > > time
> > > from enable to stable, it seems like native drivers should add
> > > some
> > > circuit-specific constants to the spec values.
> > > 
> > 
> > Reset of endpoint card via PERST# signal is defined in PCIe CEM r5
> > 2.2:
> > "On power-up, the de-assertion of PERST# is delayed 100 ms
> > (TPVPERL)
> > from the power rails achieving specified operating limits. Also,
> > within
> > this time, the reference clocks (REFCLK+, REFCLK-) also become
> > stable,
> > at least TPERST-CLK before PERST# is de-asserted."
> > 
> > - Tpvperl - PERST# must remain active at least this long after
> > power
> > becomes valid
> > 
> > Initialize steps as following(please correct me if I'm wrong):
> >  1) Enable main power
> >  2) Assert PERST#
> >  3) Sleep 100ms (TPVPERL, within this time, the REFCLK and power
> > also
> > become stable)(CEM r5 figure 8: power up)
> >  4) Deassert PERST#
> >  5) wait until link training completes by software polling the Data
> > Link Layer Link Active bit
> >  6) if speed is greater than 5GT/s, wait another 100ms
> 
> Hello! About month ago I was investigating correct order of steps and
> I
> wrote email about it, please look at it:
> 
https://lore.kernel.org/linux-pci/20211022183808.jdeo7vntnagqkg7g@pali/

Hi Pali,

Thanks for your investigating.
Modify the code to refer to your steps as following(please correct me
if I'm wrong):
1) pcie_assert_reset();
2) pcie_power_on();
3) pcie_setup_refclk();
4) msleep(100);
5) pcie_deassert_reset();
6) polling_link_active_bit();
7) msleep(100);

> 
> > > > >   - Where does mediatek enforce the PCIe sec 6.6.1 delay
> > > > > after
> > > > >     deasserting PERST# and before config requests?
> > > > > 
> > 
> > Software can determine when Link training completes by polling the
> > Data
> > Link Layer Link Active bit for maximum 100ms.
> 
> My understanding is that it is needed to wait another 100ms _after_
> link
> training completes.

I still have some doubt the two cases that the spec proposes to be
greater than 5GT/s and not greater than 5GT/s, and even divided into
two paragraphs.(spec r6.6.1)

> 
> > > > >   - Does either apple or mediatek support speeds greater than
> > > > > 5
> > > > > GT/s,
> > > > >     and if so, shouldn't we start the sec 6.6.1 100ms delay
> > > > > *after*
> > > > >     Link training completes?
> > > > 
> > > > The Apple hardware advertises support for 8 GT/s, but all the
> > > > devices
> > > > integrated on the Mac mini support only 2.5 GT/s or 5 GT/s.
> > > 
> > > The spec doesn't say anything about what the downstream devices
> > > support (obviously it can't because we don't *know* what those
> > > devices
> > > are until after we enumerate them).  So to be pedantically
> > > correct,
> > > I'd argue that we should pay attention to what the Root Port
> > > advertises.  Of course, I don't think we do this correctly
> > > *anywhere*
> > > today.
> > 
> > Thanks