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

[Verma, Achal]

On 7/18/2023 9:25 PM, Bjorn Helgaas wrote:
> 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 at 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);
> 
I think adding 100us more is not required since as you said and as also 
mentioned in CEM spec, 100ms covers for both power rails and refclock to
get stable and 2 consecutive sleep call looks different to me.
But if still required (please let me know), will do the suggested 
change, along with other fixes you asked below.
> 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.
sure will change variable name to perst_gpiod.
> 
> 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?
> 
yes gpiod_get shouldn't be optional, will fix this too.

>> 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
>>
>>
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