Re: [PATCH v2] PCI: mediatek-gen3: Avoid PCIe resetting via PCIE_RSTB for Airoha EN7581 SoC

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Nov 05, 2024 at 07:13:52PM +0100, Lorenzo Bianconi wrote:
> > On Mon, Nov 04, 2024 at 11:00:05PM +0100, Lorenzo Bianconi wrote:
> > > Airoha EN7581 has a hw bug asserting/releasing PCIE_PE_RSTB signal
> > > causing occasional PCIe link down issues. In order to overcome the
> > > problem, PCIE_RSTB signals are not asserted/released during device probe or
> > > suspend/resume phase and the PCIe block is reset using REG_PCI_CONTROL
> > > (0x88) and REG_RESET_CONTROL (0x834) registers available via the clock
> > > module.
> > > Introduce flags field in the mtk_gen3_pcie_pdata struct in order to
> > > specify per-SoC capabilities.
> > 
> > Where does this alternate way of doing reset (using REG_PCI_CONTROL
> > and REG_RESET_CONTROL) happen?  Why isn't there something in this
> > patch to use that alternate method at the same points where
> > PCIE_PE_RSTB is used?
> 
> REG_RESET_CONTROL (0x834) is already asserted/released in the following flow:
> 
> mtk_pcie_en7581_power_up() -> reset_control_bulk_deassert() -> en7523_reset_update()
> https://github.com/torvalds/linux/blob/master/drivers/clk/clk-en7523.c#L470
> 
> REG_PCI_CONTROL (0x88) is already asserted/released in the following flow:
> mtk_pcie_en7581_power_up() -> clk_bulk_enable() -> en7581_pci_enable()
> https://github.com/torvalds/linux/blob/master/drivers/clk/clk-en7523.c#L385

So IIUC, you're saying that on EN7581, the PCI hierarchy is reset by
the soc->power_up() callback, mtk_pcie_en7581_power_up(), via
REG_PCI_CONTROL and REG_RESET_CONTROL.

I assume the hierarchy is also reset by the non-EN7581 .power_up()
callback, mtk_pcie_power_up()?

And prior to this patch, we reset the hierarchy *again* in
mtk_pcie_startup_port() via PCIE_RST_CTRL_REG, but this causes
occasional "link down" issues because of a EN7581 hardware defect.

So for EN7581, this patch skips the PCIE_RST_CTRL_REG reset in
mtk_pcie_startup_port().

.power_up() and mtk_pcie_startup_port() are used both at probe time
and in mtk_pcie_resume_noirq().  So after this patch, I assume:

  - EN7581 resets the hierarchy once at probe and resume instead of
    twice.

  - Non-EN7581 resets the hierarchy twice at probe and resume.

I assume I'm missing something (maybe mtk_pcie_power_up() doesn't
actually reset the hierarchy?) because I don't see why we would reset
the hierarchy twice for either controller.

Bjorn