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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> 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.

yes, correct.

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

as pointed out by Jianjun Wang, non-EN7581 family is reset via PCIE_RSTB
signals and .power_up() callback is used just for initialization.
For EN7581 family we need to reset the device via .power_up() callback
since we have a hw issue with PCIE_PE_RSTB signal (at least this is my
take-away :))

> 
> 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.

yes, correct

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

yes, correct

> 
> .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.

yes, correct

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

nope, just once since .power_up() does not reset the device.

Regards,
Lorenzo

> 
> 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

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