Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

From: Luca Ceresoli <luca@lucaceresoli.net>
To: "Pali Rohár" <pali@kernel.org>, "Kishon Vijay Abraham I" <kishon@ti.com>
Cc: linux-pci@vger.kernel.org, linux-omap@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Rob Herring <robh@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour
Date: Tue, 22 Jun 2021 12:57:22 +0200	[thread overview]
Message-ID: <8207a53c-4de9-d0e5-295a-c165e7237e36@lucaceresoli.net> (raw)
In-Reply-To: <20210531162242.jm73yzntzmilsvbg@pali>

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