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

[qizhong.cheng <qizhong.cheng@mediatek.com>]

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

> > >   - 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.

> > >   - 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
_______________________________________________
Linux-mediatek mailing list
Linux-mediatek@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-mediatek