Re: [PATCH 5/8] cxl/acpi: Introduce ACPI0017 driver and cxl_root

From: Dan Williams <dan.j.williams@intel.com>
To: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: linux-cxl@vger.kernel.org, Linux PCI <linux-pci@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux ACPI <linux-acpi@vger.kernel.org>
Subject: Re: [PATCH 5/8] cxl/acpi: Introduce ACPI0017 driver and cxl_root
Date: Tue, 11 May 2021 23:29:32 -0700	[thread overview]
Message-ID: <CAPcyv4hhYJsTaRbCXKQaHWw8KwfOFxa-pG4x2r8HCYqVTshZCA@mail.gmail.com> (raw)
In-Reply-To: <20210510155615.000001fc@Huawei.com>

On Mon, May 10, 2021 at 7:58 AM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Fri, 7 May 2021 15:51:47 -0700
> Dan Williams <dan.j.williams@intel.com> wrote:
>
> > While CXL builds upon the PCI software model for dynamic enumeration and
> > control, a static platform component is required to bootstrap the CXL
> > memory layout. In addition to identifying the host bridges ACPI is
> > responsible for enumerating the CXL memory space that can be addressed
> > by decoders. This is similar to the requirement for ACPI to publish
> > resources reported by _CRS for PCI host bridges.
> >
> > Introduce the cxl_root object as an abstract "port" into the CXL.mem
> > address space described by HDM decoders identified by the ACPI
> > CEDT.CHBS.
> >
> > For now just establish the initial boilerplate and sysfs attributes, to
> > be followed by enumeration of the ports within the host bridge.
> >
> > Note the allocation of CXL core device objects is split into separate
> > alloc and add steps in order to separate the alloc error path (kfree())
> > from the device add error path (put_device()).
> >
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>
> Hi Dan
>
> Just one bit in here that confused me (assuming I'm reading the code correctly).
> You have is_visible for the dev_attr_supports_pmem etc to only show them if
> the particular space supports that memory type. That's fine. You also have
> the actual sysfs function checking the same flag to decide to return "1" or "0"
> which would also be fine, but in combination it's rather odd as the sysfs
> read function can never return "0" (sysfs attribute isn't visible in that
> condition). Probably deserves at least a comment.

Ok.

That was deliberate since it's trivial to code and allows the
visibility policy to change without needing to go audit the attributes
that assumed invisibility. However, yes, it deserves a comment to save
brain cycles with that "hmm, that's odd" in the future.

> This also needs some documentation for the new sysfs ABI
> (Documentation/ABI/...) but that can be in a separate patch.

True.

>
> Otherwise looks good to me.

Thanks.