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SubjectRe: [PATCH 09/15] cxl/acpi: Determine PCI host bridge's ACPI UID
On Thu, 1 Sep 2022 08:53:36 +0200
Robert Richter <rrichter@amd.com> wrote:

> On 31.08.22 12:00:27, Jonathan Cameron wrote:
> > On Wed, 31 Aug 2022 10:15:57 +0200
> > Robert Richter <rrichter@amd.com> wrote:
> >
> > > The UID is needed to read the RCH's CEDT entry with the RCRB base
> > > address. Determine the host's UID from its ACPI fw node.
> > >
> > > Signed-off-by: Robert Richter <rrichter@amd.com>
> > > ---
> > > drivers/cxl/acpi.c | 12 ++++++++++++
> > > 1 file changed, 12 insertions(+)
> > >
> > > diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> > > index f9cdf23a91a8..b3146b7ae922 100644
> > > --- a/drivers/cxl/acpi.c
> > > +++ b/drivers/cxl/acpi.c
> > > @@ -368,8 +368,20 @@ struct pci_host_bridge *cxl_find_next_rch(struct pci_host_bridge *host)
> > > static int __init cxl_restricted_host_probe(struct platform_device *pdev)
> > > {
> > > struct pci_host_bridge *host = NULL;
> > > + struct acpi_device *adev;
> > > + unsigned long long uid = ~0;
> > >
> > > while ((host = cxl_find_next_rch(host)) != NULL) {
> > > + adev = ACPI_COMPANION(&host->dev);
> > > + if (!adev || !adev->pnp.unique_id ||
> > > + (kstrtoull(adev->pnp.unique_id, 10, &uid) < 0))
> >
> > There is an acpi_device_uid() accessor function that should probably be
> > used here.
>
> That accessor actually does not help really, there is no null pointer
> check for adev. Using it actually adds more complexity since another
> variable is introduced plus you need to look at the function's
> implementation anyway.
>
> The adev->pnp.unique_id access pattern is widely used in the kernel, I
> don't expect changes in the data struct here.

Ok.

>
> > Also, should a fialure to convert to an integer (or one within
> > limits) be something we paper over? Feels like we should fail
> > hard if that happens.
>
> This is a real corner case and close to a broken firmware
> implementation. I think current dbg messages are good to find where
> the detection stops.

Hmm. I don't like papering over such bugs as it leads to people not
fixing their bios as early as they otherwise would,
but fair enough I guess.

>
> > Admittedly I can't immediately find any spec that states that
> > the _UID should be either integer or under 32 bits...
> > ACPI allows a string and CXL just says it's 4 bytes long.
>
> IIRC the UID can be implemented as string or 8 bytes, there is no
> limitation then. That's why the range check below.
Ok.

All queries answered so

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

>
> -Robert
>
> >
> > > + continue;
> > > +
> > > + dev_dbg(&adev->dev, "host uid: %llu\n", uid);
> > > +
> > > + if (uid > U32_MAX)
> > > + continue;
> > > +
> > > dev_info(&host->dev, "host supports CXL\n");
> > > }
> > >
> >

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