Re: [PATCH v3] PCI: j721e: Delay 100ms T_PVPERL from power stable to PERST# inactive

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jul 07, 2023 at 03:21:19PM +0530, Achal Verma wrote:
> As per the PCIe Card Electromechanical specification REV. 5.0, PERST#
> signal should be de-asserted after minimum 100ms from the time power-rails
> become stable. So, to ensure 100ms delay to give sufficient time for
> power-rails and refclk to become stable, change delay from 100us to 100ms.
> 
> From PCIe Card Electromechanical specification REV. 5.0 section 2.9.2:
> TPVPERL: Power stable to PERST# inactive - 100ms
> 
> Fixes: f3e25911a430 ("PCI: j721e: Add TI J721E PCIe driver")
> Signed-off-by: Achal Verma <a-verma1@ti.com>
> ---
> 
> Changes from v2:
> * Fix commit message.
> 
> Change from v1:
> * Add macro for delay value.
> 
>  drivers/pci/controller/cadence/pci-j721e.c | 11 +++++------
>  drivers/pci/pci.h                          |  2 ++
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pci/controller/cadence/pci-j721e.c b/drivers/pci/controller/cadence/pci-j721e.c
> index e70213c9060a..32b6a7dc3cff 100644
> --- a/drivers/pci/controller/cadence/pci-j721e.c
> +++ b/drivers/pci/controller/cadence/pci-j721e.c
> @@ -498,14 +498,13 @@ static int j721e_pcie_probe(struct platform_device *pdev)
>  
>  		/*
>  		 * "Power Sequencing and Reset Signal Timings" table in
> -		 * PCI EXPRESS CARD ELECTROMECHANICAL SPECIFICATION, REV. 3.0
> -		 * indicates PERST# should be deasserted after minimum of 100us
> -		 * once REFCLK is stable. The REFCLK to the connector in RC
> -		 * mode is selected while enabling the PHY. So deassert PERST#
> -		 * after 100 us.
> +		 * PCI EXPRESS CARD ELECTROMECHANICAL SPECIFICATION, REV. 5.0
> +		 * indicates PERST# should be deasserted after minimum of 100ms
> +		 * after power rails achieve specified operating limits and
> +		 * within this period reference clock should also become stable.

I think the problem is not that the current code is *wrong*, because
we do need to observe T_PERST-CLK, but that it failed to *also*
account for T_PVPERL.

There are two delays before deasserting PERST#:

  T_PVPERL: delay after power becomes stable
  T_PERST-CLK: delay after REFCLK becomes stable

I assume power is enabled by phy_power_on(), and REFCLK is enabled by
clk_prepare_enable():

  cdns_pcie_init_phy
    cdns_pcie_enable_phy
      phy_power_on             <-- power becomes stable
  clk_prepare_enable           <-- REFCLK becomes stable
  if (gpiod)
    usleep_range
    gpiod_set_value_cansleep(gpiod, 1)   <-- deassert PERST#

I don't actually know if phy_power_on() guarantees that power is
stable before it returns.  But I guess that's our assumption?
Similarly for clk_prepare_enable().

In any case, we have to observe both delays.  They overlap, and
T_PVPERL is 1000 times longer than T_PERST-CLK, so there might be
enough slop in an msleep(100) to cover both, but I think I would do
the simple-minded:

  msleep(PCIE_TPVPERL_MS);
  usleep_range(PCIE_TPERST_CLK_US, 2 * PCIE_TPERST_CLK_US);

This is slightly more conservative than necessary because they
overlap, but at least it shows that we thought about both of them.

>  		if (gpiod) {
> -			usleep_range(100, 200);
> +			msleep(PCIE_TPVPERL_DELAY_MS);
>  			gpiod_set_value_cansleep(gpiod, 1);

I wish this local variable were named something like "perst_gpiod"
instead of "gpiod".  We already know from its use in
gpiod_set_value_cansleep() that it's a GPIO.  What's NOT obvious from
the context is that this is the PERST# signal.

Tangent: it looks like the DT "reset" property that I'm assuming
controls PERST# is optional.  How do we enforce these delays if that
property is missing?

> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index a4c397434057..6ab2367e5867 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -13,6 +13,8 @@
>  
>  #define PCIE_LINK_RETRAIN_TIMEOUT_MS	1000
>  
> +#define PCIE_TPVPERL_DELAY_MS	100	/* see PCIe CEM r5.0, sec 2.9.2 */
> +
>  extern const unsigned char pcie_link_speed[];
>  extern bool pci_early_dump;
>  
> -- 
> 2.25.1
> 
> 
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel