linux-kernel - Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

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Date:   Tue, 22 Jun 2021 12:57:22 +0200
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     Pali Rohár <pali@...nel.org>,
        Kishon Vijay Abraham I <kishon@...com>
Cc:     linux-pci@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

Hi,

On 31/05/21 18:22, Pali Rohár wrote:
> Hello Kishon!
> 
> On Monday 31 May 2021 21:30:30 Kishon Vijay Abraham I wrote:
>> I had given the timing mentioned in the specification here
>> https://lore.kernel.org/r/023c9b59-70bb-ed8d-a4c0-76eae726b574@ti.com
>>
>> The PCI EXPRESS CARD ELECTROMECHANICAL SPECIFICATION defines the Power
>> Sequencing and Reset Signal Timings in Table 2-4. Please also refer Figure
>> 2-10: Power Up of the CEM.
>>
>> ╔═════════════╤══════════════════════════════════════╤═════╤═════╤═══════╗
>> ║ Symbol      │ Parameter                            │ Min │ Max │ Units ║
>> ╠═════════════╪══════════════════════════════════════╪═════╪═════╪═══════╣
>> ║ T PVPERL    │ Power stable to PERST# inactive      │ 100 │     │ ms    ║
>> ╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
>> ║ T PERST-CLK │ REFCLK stable before PERST# inactive │ 100 │     │ μs    ║
>> ╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
>> ║ T PERST     │ PERST# active time                   │ 100 │     │ μs    ║
>> ╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
>> ║ T FAIL      │ Power level invalid to PERST# active │     │ 500 │ ns    ║
>> ╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
>> ║ T WKRF      │ WAKE# rise – fall time               │     │ 100 │ ns    ║
>> ╚═════════════╧══════════════════════════════════════╧═════╧═════╧═══════╝
>>
>> The de-assertion of #PERST is w.r.t both power stable and refclk stable.
> 
> I think this does not fully answer this problematic question. One thing
> is initial power on and second thing is warm reset (when both power and
> clock is stable).
> 
> On more ARM boards, power is not SW controllable and is automatically
> enabled when powering board on. So Tₚᵥₚₑᵣₗ is calculated since
> bootloader and therefore not needed to take into account in kernel.
> 
> Tₚₑᵣₛₜ₋cₗₖ is only 100 µs and experiments proved that 100 µs not enough
> for toggling PERST# GPIO. At least one 1 ms is needed and for some cards
> at least 10 ms. Otherwise cards are not detected.
> 
> So when you have both power and clock stable and you want to reset card
> via PERST# signal, above table does not say how long it is needed to
> have PERST# in reset state.

Nothing happened after a few weeks... I understand that knowing the
correct reset timings is relevant, but unfortunately I cannot help much
in finding out the correct values.

However I'm wondering what should happen to this patch. It *does* fix a
real bug, but potentially with an incorrect or non-optimal usleep range.
Do we really want to ignore a bugfix because we are not sure about how
long this delay should be?

-- 
Luca