linux-kernel - Re: [PATCH 1/3] cxl/mem: Fix register block offset calculation

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Date:   Thu, 15 Apr 2021 17:03:17 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ben Widawsky <ben.widawsky@...el.com>
Cc:     linux-cxl@...r.kernel.org, Linux PCI <linux-pci@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        "Schofield, Alison" <alison.schofield@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] cxl/mem: Fix register block offset calculation

On Thu, Apr 15, 2021 at 4:26 PM Ben Widawsky <ben.widawsky@...el.com> wrote:
>
> The offset for the register block should be a 64K aligned value, and
> therefore FIELD_GET (which will shift) is not correct for the
> calculation.
>
> From 8.1.9.1 of the CXL 2.0 spec:
>   A[31:16] of offset from the address contained by one of the Function's
>   Base Address Registers to point to the base of the Register Block.
>   Register Block Offset is 64K aligned. Hence A[15:0] is zero
>
> Fix this by simply using a mask.

The above reads slightly funny to me, is this any clearer?

The "Register Offset Low" register of a "DVSEC Register Locator"
contains the 64K aligned offset for the registers along with the BAR
indicator and an id. The implementation was treating the "Register
Block Offset Low" field a value rather than as a pre-aligned component
of the 64-bit offset. So, just mask, don't mask and shift (FIELD_GET).

>
> This wasn't found earlier because the primary development done in the
> QEMU environment only uses 0 offsets
>
> Fixes: 8adaf747c9f0b ("cxl/mem: Find device capabilities")

As I've learned, linux-next will flag this as the wrong format.

Fixes: 8adaf747c9f0 ("cxl/mem: Find device capabilities")

...i.e. looks like your core.abbrev setting is 13 rather than 12 per
Documentation/process/submitting-patches.rst

Other than that, fix looks good to me.

> Reported-by: Vishal Verma <vishal.l.verma@...el.com>
> Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>
> ---
>  drivers/cxl/mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> index e3003f49b329..1b5078311f7d 100644
> --- a/drivers/cxl/mem.c
> +++ b/drivers/cxl/mem.c
> @@ -998,7 +998,7 @@ static struct cxl_mem *cxl_mem_create(struct pci_dev *pdev, u32 reg_lo,
>                 return NULL;
>         }
>
> -       offset = ((u64)reg_hi << 32) | FIELD_GET(CXL_REGLOC_ADDR_MASK, reg_lo);
> +       offset = ((u64)reg_hi << 32) | (reg_lo & CXL_REGLOC_ADDR_MASK);
>         bar = FIELD_GET(CXL_REGLOC_BIR_MASK, reg_lo);
>
>         /* Basic sanity check that BAR is big enough */
> --
> 2.31.1