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

["Pali Rohár" <pali@kernel.org>]

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@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@mediatek.com>
> > > > > Acked-by: Pali Rohár <pali@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/

> > > >   - 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.

> > > >   - 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