linux-kernel - Re: [RFC PATCH v3 04/16] cxl/mem: Introduce a driver for CXL-2.0-Type-3 endpoints

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Date:   Tue, 12 Jan 2021 12:06:30 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     Ben Widawsky <ben.widawsky@...el.com>, linux-cxl@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        "linux-acpi@...r.kernel.org, Ira Weiny" <ira.weiny@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Jon Masters <jcm@...masters.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        daniel.lll@...baba-inc.com
Subject: Re: [RFC PATCH v3 04/16] cxl/mem: Introduce a driver for
 CXL-2.0-Type-3 endpoints

On Tue, Jan 12, 2021 at 11:03 AM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
>
> On Mon, 11 Jan 2021 14:51:08 -0800
> Ben Widawsky <ben.widawsky@...el.com> wrote:
>
> > From: Dan Williams <dan.j.williams@...el.com>
> >
> > The CXL.mem protocol allows a device to act as a provider of "System
> > RAM" and/or "Persistent Memory" that is fully coherent as if the memory
> > was attached to the typical CPU memory controller.
> >
> > With the CXL-2.0 specification a PCI endpoint can implement a "Type-3"
> > device interface and give the operating system control over "Host
> > Managed Device Memory". See section 2.3 Type 3 CXL Device.
> >
> > The memory range exported by the device may optionally be described by
> > the platform firmware memory map, or by infrastructure like LIBNVDIMM to
> > provision persistent memory capacity from one, or more, CXL.mem devices.
> >
> > A pre-requisite for Linux-managed memory-capacity provisioning is this
> > cxl_mem driver that can speak the mailbox protocol defined in section
> > 8.2.8.4 Mailbox Registers.
> >
> > For now just land the driver boiler-plate and fill it in with
> > functionality in subsequent commits.
> >
> > Link: https://www.computeexpresslink.org/download-the-specification
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> > Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>
>
> Just one passing comment inline.
>
> > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > new file mode 100644
> > index 000000000000..005404888942
> > --- /dev/null
> > +++ b/drivers/cxl/mem.c
> > @@ -0,0 +1,69 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/* Copyright(c) 2020 Intel Corporation. All rights reserved. */
> > +#include <linux/module.h>
> > +#include <linux/pci.h>
> > +#include <linux/io.h>
> > +#include "acpi.h"
> > +#include "pci.h"
> > +
> > +static int cxl_mem_dvsec(struct pci_dev *pdev, int dvsec)
>
> Is it worth pulling this out to a utility library now as we are going
> to keep needing this for CXL devices?
> Arguably, with a vendor_id parameter it might make sense to have
> it as a utility function for pci rather than CXL alone.

Sure, cxl_mem_dvsec() can move to a central location, but I'd wait for
the first incremental user to split it out.