linux-kernel - Re: [PATCH v11 11/20] cxl/pci: Add RCH downstream port AER register discovery

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Message-ID: <20231002155334.00004e58@Huawei.com>
Date:   Mon, 2 Oct 2023 15:53:34 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Robert Richter <rrichter@....com>
CC:     Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Davidlohr Bueso" <dave@...olabs.net>,
        Dave Jiang <dave.jiang@...el.com>, <linux-cxl@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Terry Bowman <terry.bowman@....com>
Subject: Re: [PATCH v11 11/20] cxl/pci: Add RCH downstream port AER register
 discovery

On Wed, 27 Sep 2023 17:43:30 +0200
Robert Richter <rrichter@....com> wrote:

> From: Terry Bowman <terry.bowman@....com>
> 
> Restricted CXL host (RCH) downstream port AER information is not currently
> logged while in the error state. One problem preventing the error logging
> is the AER and RAS registers are not accessible. The CXL driver requires
> changes to find RCH downstream port AER and RAS registers for purpose of
> error logging.
> 
> RCH downstream ports are not enumerated during a PCI bus scan and are
> instead discovered using system firmware, ACPI in this case.[1] The
> downstream port is implemented as a Root Complex Register Block (RCRB).
> The RCRB is a 4k memory block containing PCIe registers based on the PCIe
> root port.[2] The RCRB includes AER extended capability registers used for
> reporting errors. Note, the RCH's AER Capability is located in the RCRB
> memory space instead of PCI configuration space, thus its register access
> is different. Existing kernel PCIe AER functions can not be used to manage
> the downstream port AER capabilities and RAS registers because the port was
> not enumerated during PCI scan and the registers are not PCI config
> accessible.
> 
> Discover RCH downstream port AER extended capability registers. Use MMIO
> accesses to search for extended AER capability in RCRB register space.
> 
> [1] CXL 3.0 Spec, 9.11.2 - System Firmware View of CXL 1.1 Hierarchy
> [2] CXL 3.0 Spec, 8.2.1.1 - RCH Downstream Port RCRB
> 
> Co-developed-by: Robert Richter <rrichter@....com>
> Signed-off-by: Terry Bowman <terry.bowman@....com>
> Signed-off-by: Robert Richter <rrichter@....com>

This doesn't look right. IIRC Co-dev tag should be just
before the SoB.

> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Reviewed-by: Dave Jiang <dave.jiang@...el.com>

New day, fresh questions....


> ---
>  drivers/cxl/core/core.h |  1 +
>  drivers/cxl/core/pci.c  |  6 ++++++
>  drivers/cxl/core/regs.c | 35 +++++++++++++++++++++++++++++++++++
>  3 files changed, 42 insertions(+)
> 
> diff --git a/drivers/cxl/core/core.h b/drivers/cxl/core/core.h
> index 45e7e044cf4a..f470ef5c0a6a 100644
> --- a/drivers/cxl/core/core.h
> +++ b/drivers/cxl/core/core.h
> @@ -73,6 +73,7 @@ struct cxl_rcrb_info;
>  resource_size_t __rcrb_to_component(struct device *dev,
>  				    struct cxl_rcrb_info *ri,
>  				    enum cxl_rcrb which);
> +u16 cxl_rcrb_to_aer(struct device *dev, resource_size_t rcrb);
>  
>  extern struct rw_semaphore cxl_dpa_rwsem;
>  
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index 6ba3b7370816..4c6c5c7ba5a3 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -722,6 +722,12 @@ static bool cxl_report_and_clear(struct cxl_dev_state *cxlds)
>  
>  void devm_cxl_setup_parent_dport(struct device *host, struct cxl_dport *dport)
>  {
> +	struct device *dport_dev = dport->dport_dev;
> +	struct pci_host_bridge *host_bridge;
> +
> +	host_bridge = to_pci_host_bridge(dport_dev);
> +	if (host_bridge->native_cxl_error)
> +		dport->rcrb.aer_cap = cxl_rcrb_to_aer(dport_dev, dport->rcrb.base);
>  }
>  EXPORT_SYMBOL_NS_GPL(devm_cxl_setup_parent_dport, CXL);
>  
> diff --git a/drivers/cxl/core/regs.c b/drivers/cxl/core/regs.c
> index e0fbe964f6f0..6e502f02899b 100644
> --- a/drivers/cxl/core/regs.c
> +++ b/drivers/cxl/core/regs.c
> @@ -470,6 +470,41 @@ int cxl_setup_regs(struct cxl_register_map *map)
>  }
>  EXPORT_SYMBOL_NS_GPL(cxl_setup_regs, CXL);
>  
> +u16 cxl_rcrb_to_aer(struct device *dev, resource_size_t rcrb)
> +{
> +	void __iomem *addr;
> +	u16 offset = 0;
> +	u32 cap_hdr;
> +
> +	if (WARN_ON_ONCE(rcrb == CXL_RESOURCE_NONE))
> +		return 0;
> +
> +	if (!request_mem_region(rcrb, SZ_4K, dev_name(dev)))
> +		return 0;
> +
> +	addr = ioremap(rcrb, SZ_4K);
> +	if (!addr) {

Given this handling exists, below, perhaps a goto?
Also, why isn't this an error?  A comment would be good for that.

> +		release_mem_region(rcrb, SZ_4K);
> +		return 0;
> +	}
> +
> +	cap_hdr = readl(addr + offset);
> +	while (PCI_EXT_CAP_ID(cap_hdr) != PCI_EXT_CAP_ID_ERR) {
> +		offset = PCI_EXT_CAP_NEXT(cap_hdr);
> +		if (!offset)
> +			break;
> +		cap_hdr = readl(addr + offset);
> +	}
> +
> +	if (offset)

Add a comment / specification reference for why an offset of 0 is not valid.
Of the top of my head I'm not sure though there may be a requirement for
something else coming first...

> +		dev_dbg(dev, "found AER extended capability (0x%x)\n", offset);
> +
> +	iounmap(addr);
> +	release_mem_region(rcrb, SZ_4K);
> +
> +	return offset;
> +}
> +
>  resource_size_t __rcrb_to_component(struct device *dev, struct cxl_rcrb_info *ri,
>  				    enum cxl_rcrb which)
>  {