RE: [PATCH v4] PCI: imx6: Fix i.MX6Q/DL boot hang caused by improper PHY power sequencing

> -----Original Message-----
> From: Bjorn Helgaas <[email protected]>
> Sent: Tuesday, July 28, 2026 5:42 AM
> To: Hongxing Zhu (OSS) <[email protected]>
> Cc: Frank Li <[email protected]>; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; [email protected];
> [email protected]; [email protected]; linux-arm-
> [email protected]; [email protected]; [email protected];
> Hongxing Zhu <[email protected]>; Leonardo Costa
> <[email protected]>; Leonardo Costa <[email protected]>
> Subject: Re: [PATCH v4] PCI: imx6: Fix i.MX6Q/DL boot hang caused by improper
> PHY power sequencing
> 
> On Wed, Jul 22, 2026 at 03:10:30PM +0800, [email protected] wrote:
> > From: Richard Zhu <[email protected]>
> >
> > Commit 610fa91d9863 ("PCI: imx6: Assert PERST# before enabling
> > regulators") introduced a boot hang on i.MX6Q/DL variants by
> > reordering the initialization sequence, which exposed a critical PHY power
> control issue.
> >
> > Root cause:
> > Before commit 610fa91d9863 ("PCI: imx6: Assert PERST# before enabling
> > regulators"), the sequence was:
> >   1. imx_pcie_assert_core_reset() - asserts TEST_PD, asserts REF_CLK_EN
> >   2. imx_pcie_clk_enable() - clears TEST_PD, asserts REF_CLK_EN
> >   3. Link training starts with TEST_PD properly cleared ✓
> >
> > After commit 610fa91d9863 ("PCI: imx6: Assert PERST# before enabling
> > regulators"), the sequence became:
> >   1. imx_pcie_clk_enable() - clears TEST_PD, asserts REF_CLK_EN
> >   2. imx_pcie_assert_core_reset() - re-asserts TEST_PD, asserts REF_CLK_EN
> >   3. imx_pcie_deassert_core_reset() - does NOT clear TEST_PD
> >   4. Link training starts with TEST_PD still asserted ✗
> >
> > The reordering caused TEST_PD to be cleared prematurely in
> > clk_enable(), then re-asserted by assert_core_reset(), and never
> > cleared again before link training, resulting in the boot hang.
> 
> I intend to apply this patch for v7.2, but I have a separate question about why the
> link training failure causes a boot *hang*, not just a boot where we couldn't
> enumerate any PCIe devices.  It seems like the link training should time out, and
> we could emit a message (if
> desired) and continue without the PCIe controller.[] 
Before link training, the imx_setup_phy_mpll() function performs PHY
read/write operations for i.MX6Q/DL PCIe. These operations cause a boot hang
when the PHY is not powered up properly, which occurs before we even reach the
link training timeout mechanism.

Best Regards
Richard Zhu