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

[Bjorn Helgaas <helgaas@kernel.org>]

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>

> > ...
> > 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.

> >   - Where does mediatek enforce the PCIe sec 6.6.1 delay after
> >     deasserting PERST# and before config requests?
> > 
> >   - 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.

Bjorn

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