Re: [PATCH v2 3/3] PCI: imx: Initial imx7d pm support

[Lucas Stach <l.stach@pengutronix.de>]

Am Dienstag, den 24.07.2018, 12:04 +0000 schrieb Leonard Crestez:
> On Tue, 2018-07-24 at 12:09 +0200, Lucas Stach wrote:
> > Am Montag, den 23.07.2018, 12:37 +0000 schrieb Leonard Crestez:
> > > On Mon, 2018-07-23 at 11:38 +0200, Lucas Stach wrote:
> > > > Am Freitag, den 20.07.2018, 15:47 +0300 schrieb Leonard Crestez:
> > > > > On imx7d the pcie-phy power domain is turned off in suspend and this can
> > > > > make the system hang after resume when attempting any read from PCI.
> > > > > 
> > > > > Fix this by adding minimal suspend/resume code from the nxp internal
> > > > > tree. This will prepare for powering down on suspend and reset the block
> > > > > on resume.
> > > > > 
> > > > > Code is only for imx7d but a very similar sequence can be used for
> > > > > other socs.
> > > > > 
> > > > > +static void imx6_pcie_ltssm_disable(struct device *dev)
> > > > > +{
> > > > > > > > > > +	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
> > > > > +
> > > > > > > > > > +	switch (imx6_pcie->variant) {
> > > > > > > > > > +	case IMX6Q:
> > > > > > > > > > +	case IMX6SX:
> > > > > > > > > > +	case IMX6QP:
> > > > > > > > > > +		regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR12,
> > > > > +				   IMX6Q_GPR12_PCIE_CTL_2, 0);
> > > > 
> > > > Has this been tested on i.MX6? LTSSM disable requires a more complex
> > > > sequence on this SoC to avoid hanging the system. See commit
> > > > 3e3e406e3807 "PCI: imx6: Put LTSSM in "Detect" state before disabling
> > > > it".
> > > 
> > > This patch only enables suspend/resume for imx7d with other SOCs to
> > > follow later. The ltssm_disable function is just symmetric with
> > > ltssm_enable.
> > > 
> > > The 6Q parts are affected by errata "ERR005723 PCIe: PCIe does not
> > > support L2 power down [i.MX 6Dual/6Quad Only]".
> > > 
> > > This design error seems to have the same root cause as your problem (no
> > > dedicated reset control) so this works out quite nicely: the solution
> > > is to never power down pci on affected chips.
> > 
> > I don't quite like code that looks like it is doing the right thing,
> > but then doesn't. Can we at least emit a warning that there might be
> > dragons if anyone tries to call this on i.MX6?
> 
> But the function will indeed toggle the right bits to initiate ltssm
> disabling. If this is not useful or can't be used right then it's the
> caller's problem :)

I don't agree with that. I would expect a function that is called
ltssm_disable to do so in a way that is safe on the hardware that it
claims to handle, which in case of i.MX6 means bashing the LTSSM into
detect state before toggling the GPR bit.

> I can add a default: clause to the switch which returns -ENOSYS and let
> IMX6Q fall that way, would that be OK? This would also help when adding
> new variants.

Yes, given that there is currently no way to test this on i.MX6,
returning -ENOSYS sound much better. This at least tells anyone who
intends to use this function that there is indeed missing functionality
there.

Regards,
Lucas