linux-kernel - Re: [PATCH v3 1/6] phy: add a driver for the Berlin SATA PHY

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Message-ID: <5764751.ffUj9rhnMd@wuerfel>
Date:	Wed, 14 May 2014 20:12:17 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Cc:	Antoine Ténart 
	<antoine.tenart@...e-electrons.com>,
	linux-arm-kernel@...ts.infradead.org,
	thomas.petazzoni@...e-electrons.com, zmxu@...vell.com,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	kishon@...com, linux-ide@...r.kernel.org,
	alexandre.belloni@...e-electrons.com, jszhang@...vell.com,
	tj@...nel.org
Subject: Re: [PATCH v3 1/6] phy: add a driver for the Berlin SATA PHY

On Wednesday 14 May 2014 19:57:46 Sebastian Hesselbarth wrote:
> On 05/14/2014 06:57 PM, Antoine Ténart wrote:
> > On Wed, May 14, 2014 at 06:11:24PM +0200, Arnd Bergmann wrote:
> >> On Wednesday 14 May 2014 17:49:29 Antoine Ténart wrote:
> >>> On Wed, May 14, 2014 at 05:31:24PM +0200, Arnd Bergmann wrote:
> 
> From what I understand from the conversation, we have a single PHY
> register set dealing with both SATA ports available on the SoC.
> Also, from the name of the PHY bits we assume the PHY may be able
> to work in different modes than just SATA. And we currently have
> an AHCI-compatible SATA IP that supports up to two ports, with one
> actually connected to a SATA plug on the DMP board.
> 
> Now, thinking about the PHY binding and the (possible) multi-protocol
> support, it can be possible that on BG2Q there is a generic 2-lane
> LVDS PHY that can be configured to support SATA or PCIe. Both are
> electrically and bit-level compatible, so they could be internally
> wired-up with AHCI and PCIe controller.

Sounds like a reasonable guess. We have other PHY drivers doing the
same thing already.

> From a DT point-of-view, we need a way to (a) link each SATA or PCIe
> port to the PHY, (b) specify the PHY lane to be used, and (c) specify
> the protocol to be used on that lane. If I got it right, Arnd already
> mentioned to use the phy-specifier to deal with it:
> 
> e.g. phy = <&genphy 0 MODE_SATA> or phy = <&genphy 1 MODE_PCIE>

Right.

> Let's assume we have one dual-port SATA controller and one PCIe
> controller with either x1 or x2 support. The only sane DT binding,
> I can think of then would be:
> 
> berlin2q.dtsi:
> 
> genphy: lvds@...0ff {
> 	compatible = "marvell,berlin-lvds-phy";
> 	reg = <0xea00ff 0x100>;
> 	#phy-cells = <2>;
> };
> 
> sata: sata@...0ff {
> 	compatible = "ahci-platform";
> 	reg = <0xab00ff 0x100>;
> 	
> 	sata0: sata-port@0 {
> 		reg = <0>;
> 		phy = <&genphy 0 MODE_SATA>;
> 		status = "disabled";
> 	};
> 
> 	sata1: sata-port@1 {
> 		reg = <1>;
> 		phy = <&genphy 1 MODE_SATA>;
> 		status = "disabled";
> 	};
> };
> 
> pcie: pcie@...1ff {
> 	compatible = "marvell,berlin-pcie";
> 	reg = <0xab01ff 0x100>;
> 
> 	pcie0: pcie-port@0 {
> 		reg = <0>;
> 		/* set phy on a per-board basis */
> 		/* PCIe x1 on Lane 0 : phy = <&genphy 0 MODE_PCIE>; */
> 		/* PCIe x2 on Lane 0 and 1 : phy = <&genphy 0 MODE_PCIE>, <&genphy 1
> MODE_PCIE>; */
> 		status = "disabled";
> 	};
> };
> 
> berlin2q-dmp.dts:
> 
> &sata1 {
> 	status = "okay";
> };
> 
> &pcie0 {
> 	phy = <&genphy 1 MODE_PCIE>;
> };
> 
> berlin2q-foo.dts:
> 
> &pcie0 {
> 	phy = <&genphy 0 MODE_PCIE>, <&genphy 1 MODE_PCIE>;
> };

Exactly. I would also be fine with keeping the sub-nodes of the
phy device as in v3 and using #phy-cells=<1> instead of #phy-cells.
The result would be pretty much the same, it just depends on how
closely connected the two logical phys are.

IIRC for the ST microelectronics PHY we recently reviewed, the
same PHY could be driving multiple SATA ports, or a single
multi-lane PCIe. In that case, it makes more sense to have
#phy-cells=<2>, like your example.
 
> For the driver, Antoine then would have to squeeze all PHY register
> mangling in phy-berlin2.c and see how to make ahci-platform aware of
> individual port nodes (I haven't looked up if it already exists, sorry)
> and announce only enabled port child nodes, right?

I've been thinking some more about this aspect. I don't actually have
a strong opinion on whether it's better to use the generic ahci-platform
driver, or to keep the multi-phy support as a special variant for
berlin. If we do the latter, it would however be good to define the
binding in a way that lets us later merge things into the generic phy
driver in case we get more of the same.

	Arnd
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