linux-kernel - Re: [RFC PATCH 2/2] PCI: apple: Add driver for the Apple M1

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Message-ID: <871r6u1wlz.wl-maz@kernel.org>
Date:   Sun, 15 Aug 2021 17:49:44 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     "Sven Peter" <sven@...npeter.dev>
Cc:     "Alyssa Rosenzweig" <alyssa@...enzweig.io>,
        linux-pci@...r.kernel.org, "Bjorn Helgaas" <bhelgaas@...gle.com>,
        "Rob Herring" <robh+dt@...nel.org>,
        "Lorenzo Pieralisi" <lorenzo.pieralisi@....com>,
        Krzysztof Wilczyński 
        <kw@...ux.com>, "Stan Skowronek" <stan@...ellium.com>,
        "Mark Kettenis" <kettenis@...nbsd.org>,
        "Hector Martin" <marcan@...can.st>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] PCI: apple: Add driver for the Apple M1

On Sun, 15 Aug 2021 13:33:14 +0100,
"Sven Peter" <sven@...npeter.dev> wrote:
> 
> 
> 
> On Sun, Aug 15, 2021, at 09:42, Marc Zyngier wrote:
> > On Sun, 15 Aug 2021 05:25:25 +0100,
> > Alyssa Rosenzweig <alyssa@...enzweig.io> wrote:
> [...]
> > > +
> > > +static int apple_pcie_setup_port(struct apple_pcie *pcie, unsigned int i)
> > > +{
> > > +	struct fwnode_handle *fwnode = dev_fwnode(pcie->dev);
> > > +	void __iomem *port;
> > > +	struct gpio_desc *reset;
> > > +	uint32_t stat;
> > > +	int ret;
> > > +
> > > +	port = devm_of_iomap(pcie->dev, to_of_node(fwnode), i + 3, NULL);
> > > +
> > > +	if (IS_ERR(port))
> > > +		return -ENODEV;
> > > +
> > > +	reset = devm_gpiod_get_index(pcie->dev, "reset", i, 0);
> > > +	if (IS_ERR(reset))
> > > +		return PTR_ERR(reset);
> > > +
> > > +	gpiod_direction_output(reset, 0);
> > > +
> > > +	rmwl(0, PORT_APPCLK_EN, port + PORT_APPCLK);
> > > +
> > > +	ret = apple_pcie_setup_refclk(pcie->rc, port, i);
> > > +	if (ret < 0)
> > > +		return ret;
> > > +
> > > +	rmwl(0, PORT_PERST_OFF, port + PORT_PERST);
> > > +	gpiod_set_value(reset, 1);
> > > +
> > > +	ret = readl_poll_timeout(port + PORT_STATUS, stat,
> > > +				 stat & PORT_STATUS_READY, 100, 250000);
> > > +	if (ret < 0) {
> > > +		dev_err(pcie->dev, "port %u ready wait timeout\n", i);
> > > +		return ret;
> > > +	}
> > > +
> > > +	rmwl(PORT_REFCLK_CGDIS, 0, port + PORT_REFCLK);
> > > +	rmwl(PORT_APPCLK_CGDIS, 0, port + PORT_APPCLK);
> > > +
> > > +	ret = readl_poll_timeout(port + PORT_LINKSTS, stat,
> > > +				 !(stat & PORT_LINKSTS_BUSY), 100, 250000);
> > > +	if (ret < 0) {
> > > +		dev_err(pcie->dev, "port %u link not busy timeout\n", i);
> > > +		return ret;
> > > +	}
> > > +
> > > +	writel(0xfb512fff, port + PORT_INTMSKSET);
> > 
> > Magic. What is this for?
> 
> The magic comes from the original Corellium driver. It first masks everything
> except for the interrupts in the next line, then acks the interrupts it keeps
> enabled and then probably wants to wait for PORT_INT_LINK_UP (or any of the
> other interrupts which seem to indicate various error conditions) to fire but
> instead polls for PORT_LINKSTS_UP.
> 
> > 
> > > +
> > > +	writel(PORT_INT_LINK_UP | PORT_INT_LINK_DOWN | PORT_INT_AF_TIMEOUT |
> > > +	       PORT_INT_REQADDR_GT32 | PORT_INT_MSI_ERR |
> > > +	       PORT_INT_MSI_BAD_DATA | PORT_INT_CPL_ABORT |
> > > +	       PORT_INT_CPL_TIMEOUT | (1 << 26), port + PORT_INTSTAT);
> > > +
> > > +	usleep_range(5000, 10000);
> > > +
> > > +	rmwl(0, PORT_LTSSMCTL_START, port + PORT_LTSSMCTL);
> > > +
> > > +	ret = readl_poll_timeout(port + PORT_LINKSTS, stat,
> > > +				 stat & PORT_LINKSTS_UP, 100, 500000);
> > > +	if (ret < 0) {
> > > +		dev_err(pcie->dev, "port %u link up wait timeout\n", i);
> > > +		return ret;
> > > +	}
> > 
> > I have the strong feeling that a lot of things in the above is to get
> > an interrupt when the port reports an event. Why the polling then?
> 
> 
> I'm pretty sure this is true. The same registers are also used to setup
> and handle legacy interrupts.
>
> My current understanding is that PORT_INTSTAT is used to retrieve the fired
> interrupts and ack them, and PORT_INTMSK are the masked interrupts.
> And then PORT_INTMSKSET and PORT_INTMSKCLR can be used to manipulate
> individual bits of PORT_INTMSK with a single store.

So this really should be modelled as an interrupt controller. If
someone comes up with a bit of a spec (though the bit assignment is
relatively clear), I can update the interrupt code to handle
that. After all, I moan enough at people writing horrible PCI driver
code, I might as well write an example myself and point them to it.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.