Date:   Sat, 18 Jun 2022 12:39:46 +0000
From:   Ioana Ciornei <ioana.ciornei@....com>
To:     Sean Anderson <sean.anderson@...o.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Eric Dumazet <edumazet@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kishon Vijay Abraham I <kishon@...com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Vinod Koul <vkoul@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-phy@...ts.infradead.org" <linux-phy@...ts.infradead.org>
Subject: Re: [PATCH net-next 03/28] phy: fsl: Add QorIQ SerDes driver

> > Subject: [PATCH net-next 03/28] phy: fsl: Add QorIQ SerDes driver
> >

Sorry for the previous HTML formatted email...

> 
> Hi Sean,
> 
> I am very much interested in giving this driver a go on other SoCs as well
> but at the moment I am in vacation until mid next week.
> 
> 
> > This adds support for the "SerDes" devices found on various NXP QorIQ SoCs.
> > There may be up to four SerDes devices on each SoC, each supporting up to
> > eight lanes. Protocol support for each SerDes is highly heterogeneous, with
> > each SoC typically having a totally different selection of supported
> > protocols for each lane. Additionally, the SerDes devices on each SoC also
> > have differing support. One SerDes will typically support Ethernet on most
> > lanes, while the other will typically support PCIe on most lanes.
> >
> > There is wide hardware support for this SerDes. I have not done extensive
> > digging, but it seems to be used on almost every QorIQ device, including
> > the AMP and Layerscape series. Because each SoC typically has specific
> > instructions and exceptions for its SerDes, I have limited the initial
> > scope of this module to just the LS1046A. Additionally, I have only added
> > support for Ethernet protocols. There is not a great need for dynamic
> > reconfiguration for other protocols (SATA and PCIe handle rate changes in
> > hardware), so support for them may never be added.>
> > Nevertheless, I have tried to provide an obvious path for adding support
> > for other SoCs as well as other protocols. SATA just needs support for
> > configuring LNmSSCR0. PCIe may need to configure the equalization
> > registers. It also uses multiple lanes. I have tried to write the driver
> > with multi-lane support in mind, so there should not need to be any large
> > changes. Although there are 6 protocols supported, I have only tested SGMII
> > and XFI. The rest have been implemented as described in the datasheet.
> >
> > The PLLs are modeled as clocks proper. This lets us take advantage of the
> > existing clock infrastructure. I have not given the same treatment to the
> > lane "clocks" (dividers) because they need to be programmed in-concert with
> > the rest of the lane settings. One tricky thing is that the VCO (pll) rate
> > exceeds 2^32 (maxing out at around 5GHz). This will be a problem on 32-bit
> > platforms, since clock rates are stored as unsigned longs. To work around
> > this, the pll clock rate is generally treated in units of kHz.>
> > The PLLs are configured rather interestingly. Instead of the usual direct
> > programming of the appropriate divisors, the input and output clock rates
> > are selected directly. Generally, the only restriction is that the input
> > and output must be integer multiples of each other. This suggests some kind
> > of internal look-up table. The datasheets generally list out the supported
> > combinations explicitly, and not all input/output combinations are
> > documented. I'm not sure if this is due to lack of support, or due to an
> > oversight. If this becomes an issue, then some combinations can be
> > blacklisted (or whitelisted). This may also be necessary for other SoCs
> > which have more stringent clock requirements.
> 
> 
> I didn't get a change to go through the driver like I would like, but are you
> changing the PLL's rate at runtime?
> Do you take into consideration that a PLL might still be used by a PCIe or SATA
> lane (which is not described in the DTS) and deny its rate reconfiguration
> if this happens?
> 
> I am asking this because when I added support for the Lynx 28G SerDes block what
> I did in order to support rate change depending of the plugged SFP module was
> just to change the PLL used by the lane, not the PLL rate itself.
> This is because I was afraid of causing more harm then needed for all the
> non-Ethernet lanes.
> 
> >
> > The general API call list for this PHY is documented under the driver-api
> > docs. I think this is rather standard, except that most driverts configure
> > the mode (protocol) at xlate-time. Unlike some other phys where e.g. PCIe
> > x4 will use 4 separate phys all configured for PCIe, this driver uses one
> > phy configured to use 4 lanes. This is because while the individual lanes
> > may be configured individually, the protocol selection acts on all lanes at
> > once. Additionally, the order which lanes should be configured in is
> > specified by the datasheet.  To coordinate this, lanes are reserved in
> > phy_init, and released in phy_exit.
> >
> > When getting a phy, if a phy already exists for those lanes, it is reused.
> > This is to make things like QSGMII work. Four MACs will all want to ensure
> > that the lane is configured properly, and we need to ensure they can all
> > call phy_init, etc. There is refcounting for phy_init and phy_power_on, so
> > the phy will only be powered on once. However, there is no refcounting for
> > phy_set_mode. A "rogue" MAC could set the mode to something non-QSGMII and
> > break the other MACs. Perhaps there is an opportunity for future
> > enhancement here.
> >
> > This driver was written with reference to the LS1046A reference manual.
> > However, it was informed by reference manuals for all processors with
> > MEMACs, especially the T4240 (which appears to have a "maxed-out"
> > configuration).
> >
> > Signed-off-by: Sean Anderson <sean.anderson@...o.com>
> > ---
> > This appears to be the same underlying hardware as the Lynx 28G phy
> > added in 8f73b37cf3fb ("phy: add support for the Layerscape SerDes
> > 28G"). 
> 
> The SerDes block used on L1046A (and a lot of other SoCs) is not the same
> one as the Lynx 28G that I submitted. The Lynx 28G block is only included
> on the LX2160A SoC and its variants.
> 
> The SerDes block that you are adding a driver for is the Lynx 10G SerDes,
> which is why I would suggest renaming it to phy-fsl-lynx-10g.c.
> 
> Ioana

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