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Date:   Fri, 25 Feb 2022 18:58:47 +0100
From:   Pali =?utf-8?B?Um9ow6Fy?= <pali@kernel.org>
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Rob Herring <robh+dt@kernel.org>, Andrew Lunn <andrew@lunn.ch>,
        Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        Marek =?utf-8?B?QmVow7pu?= <kabel@kernel.org>,
        Russell King <rmk+kernel@armlinux.org.uk>,
        Gregory Clement <gregory.clement@bootlin.com>,
        linux-pci@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/6] PCI: Add function for parsing
 'slot-power-limit-milliwatt' DT property
Message-ID: <20220225175847.gf6rvkamnyxjx2di@pali>
References: <20220225123051.xlsv7fkesdf5upeh@pali>
 <20220225155156.GA358965@bhelgaas>
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On Friday 25 February 2022 09:51:56 Bjorn Helgaas wrote:
> On Fri, Feb 25, 2022 at 01:30:51PM +0100, Pali Rohár wrote:
> > On Thursday 24 February 2022 14:47:15 Bjorn Helgaas wrote:
> > > On Tue, Feb 22, 2022 at 05:31:56PM +0100, Pali Rohár wrote:
> > > > Add function of_pci_get_slot_power_limit(), which parses the
> > > > 'slot-power-limit-milliwatt' DT property, returning the value in
> > > > milliwatts and in format ready for the PCIe Slot Capabilities Register.
> > > > 
> > > > Signed-off-by: Pali Rohár <pali@kernel.org>
> > > > Signed-off-by: Marek Behún <kabel@kernel.org>
> > > > Reviewed-by: Rob Herring <robh@kernel.org>
> > > > Acked-by: Bjorn Helgaas <bhelgaas@google.com>
> > > > ---
> > > >  drivers/pci/of.c  | 64 +++++++++++++++++++++++++++++++++++++++++++++++
> > > >  drivers/pci/pci.h | 15 +++++++++++
> > > >  2 files changed, 79 insertions(+)
> > > > 
> > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > > index cb2e8351c2cc..2b0c0a3641a8 100644
> > > > --- a/drivers/pci/of.c
> > > > +++ b/drivers/pci/of.c
> > > > @@ -633,3 +633,67 @@ int of_pci_get_max_link_speed(struct device_node *node)
> > > >  	return max_link_speed;
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(of_pci_get_max_link_speed);
> > > > +
> > > > +/**
> > > > + * of_pci_get_slot_power_limit - Parses the "slot-power-limit-milliwatt"
> > > > + *				 property.
> > > > + *
> > > > + * @node: device tree node with the slot power limit information
> > > > + * @slot_power_limit_value: pointer where the value should be stored in PCIe
> > > > + *			    Slot Capabilities Register format
> > > > + * @slot_power_limit_scale: pointer where the scale should be stored in PCIe
> > > > + *			    Slot Capabilities Register format
> > > > + *
> > > > + * Returns the slot power limit in milliwatts and if @slot_power_limit_value
> > > > + * and @slot_power_limit_scale pointers are non-NULL, fills in the value and
> > > > + * scale in format used by PCIe Slot Capabilities Register.
> > > > + *
> > > > + * If the property is not found or is invalid, returns 0.
> > > > + */
> > > > +u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > > +				u8 *slot_power_limit_value,
> > > > +				u8 *slot_power_limit_scale)
> > > > +{
> > > > +	u32 slot_power_limit;
> > > 
> > > Including "mw" or similar reference to the units would give a hint of
> > > how to relate the code to the spec.
> > > 
> > > > +	u8 value, scale;
> > > > +
> > > > +	if (of_property_read_u32(node, "slot-power-limit-milliwatt",
> > > > +				 &slot_power_limit))
> > > > +		slot_power_limit = 0;
> > > > +
> > > > +	/* Calculate Slot Power Limit Value and Slot Power Limit Scale */
> > > 
> > > Add a spec reference to PCIe r6.0, sec 7.5.3.9.  IIUC, this supports
> > > up to 300W, which was what r5.0 defined, but r6.0 added values up to
> > > 0xfe (600W).
> > 
> > I did not know about it and I have not seen/read r6.0.
> > 
> > It would be nice if somebody with access to r6.0 send a patch to lspci
> > utility, so we could write support for 600W based on lspci parser.
> 
> Of course, sorry!  Obviously you would have implemented them all if
> you had the spec!
> 
> Here's the info from r6.0, sec 7.5.3.9:
> 
>   Slot Power Limit Value - In combination with the Slot Power Limit
>   Scale value, specifies the upper limit on power supplied by the slot
>   (see § Section 6.9) or by other means to the adapter.
> 
>   Power limit (in Watts) is calculated by multiplying the value in
>   this field by the value in the Slot Power Limit Scale field except
>   when the Slot Power Limit Scale field equals 00b (1.0x) and Slot
>   Power Limit Value exceeds EFh, the following alternative encodings
>   are used:
> 
>     F0h   > 239 W and ≤ 250 W Slot Power Limit
>     F1h   > 250 W and ≤ 275 W Slot Power Limit
>     F2h   > 275 W and ≤ 300 W Slot Power Limit
>     F3h   > 300 W and ≤ 325 W Slot Power Limit
>     F4h   > 325 W and ≤ 350 W Slot Power Limit
>     F5h   > 350 W and ≤ 375 W Slot Power Limit
>     F6h   > 375 W and ≤ 400 W Slot Power Limit
>     F7h   > 400 W and ≤ 425 W Slot Power Limit
>     F8h   > 425 W and ≤ 450 W Slot Power Limit
>     F9h   > 450 W and ≤ 475 W Slot Power Limit
>     FAh   > 475 W and ≤ 500 W Slot Power Limit
>     FBh   > 500 W and ≤ 525 W Slot Power Limit
>     FCh   > 525 W and ≤ 550 W Slot Power Limit
>     FDh   > 550 W and ≤ 575 W Slot Power Limit
>     FEh   > 575 W and ≤ 600 W Slot Power Limit
>     FFh   Reserved for Slot Power Limit Values above 600 W
> 
>   This register must be implemented if the Slot Implemented bit is Set.
> 
>   Writes to this register also cause the Port to send the
>   Set_Slot_Power_Limit Message.

Ok, thank you!

I will send also update for lspci.

> > > > +	if (slot_power_limit == 0) {
> > > > +		value = 0x00;
> > > > +		scale = 0;
> > > > +	} else if (slot_power_limit <= 255) {
> > > > +		value = slot_power_limit;
> > > > +		scale = 3;
> > > > +	} else if (slot_power_limit <= 255*10) {
> > > > +		value = slot_power_limit / 10;
> > > > +		scale = 2;
> > > > +	} else if (slot_power_limit <= 255*100) {
> > > > +		value = slot_power_limit / 100;
> > > > +		scale = 1;
> > > > +	} else if (slot_power_limit <= 239*1000) {
> > > > +		value = slot_power_limit / 1000;
> > > > +		scale = 0;
> > > > +	} else if (slot_power_limit <= 250*1000) {
> > > > +		value = 0xF0;
> > > > +		scale = 0;
> > > > +	} else if (slot_power_limit <= 275*1000) {
> > > > +		value = 0xF1;
> > > > +		scale = 0;
> > > > +	} else {
> > > > +		value = 0xF2;
> > > > +		scale = 0;
> > > > +	}
> > > > +
> > > > +	if (slot_power_limit_value)
> > > > +		*slot_power_limit_value = value;
> > > > +
> > > > +	if (slot_power_limit_scale)
> > > > +		*slot_power_limit_scale = scale;
> > > > +
> > > > +	return slot_power_limit;
> > > 
> > > If "slot-power-limit-milliwatt" contains a value larger than can be
> > > represented in "value" and "scale", the return value will not agree
> > > with value/scale, will it?
> > 
> > In previous version 0xF2 was reserved for values above 275 W. So for me
> > it looked like a correct solution.
> > 
> > > Currently you only use the return value for a log message, so no real
> > > harm yet, other than the fact that we might print "Slot power limit
> > > 1000.0W" when the hardware will only advertise 600W available.
> > > 
> > > Also, if "slot-power-limit-milliwatt" contains something like
> > > 260000 mW (260 W), we'll return 0xF1/0, so the hardware will
> > > advertise 275 W available.
> > 
> > There is no way how to encode 260 W. It is possible only 250 W or 275 W,
> > and nothing between. I chose to round value to upper limit. What do you
> > prefer in these cases? Upper or lower limit?
> 
> I think rounding down is better.  If we round up, the slot will
> advertise more power than it can deliver, and if the device tries to
> consume the amount of power advertised, it may not work reliably.
> 
> So I think we should return encoded values that are no higher than
> what the slot can actually deliver, and the return value should match
> what Slot Capabilities advertises.
> 
> Bjorn

It makes sense.