linux-kernel - Re: [RFC PATCH v3 14/16] cxl/mem: Use CEL for enabling commands

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Message-ID: <20210114183217.0000154c@Huawei.com>
Date:   Thu, 14 Jan 2021 18:32:17 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Ben Widawsky <ben.widawsky@...el.com>
CC:     <linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-pci@...r.kernel.org>, <linux-acpi@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...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 14/16] cxl/mem: Use CEL for enabling commands

On Thu, 14 Jan 2021 10:13:40 -0800
Ben Widawsky <ben.widawsky@...el.com> wrote:

> On 21-01-14 18:02:11, Jonathan Cameron wrote:
> > On Mon, 11 Jan 2021 14:51:19 -0800
> > Ben Widawsky <ben.widawsky@...el.com> wrote:
> >   
> > > The Command Effects Log (CEL) is specified in the CXL 2.0 specification.
> > > The CEL is one of two types of logs, the other being vendor specific.
> > > They are distinguished in hardware/spec via UUID. The CEL is immediately
> > > useful for 2 things:
> > > 1. Determine which optional commands are supported by the CXL device.
> > > 2. Enumerate any vendor specific commands
> > > 
> > > The CEL can be used by the driver to determine which commands are
> > > available in the hardware (though it isn't, yet). That set of commands
> > > might itself be a subset of commands which are available to be used via
> > > CXL_MEM_SEND_COMMAND IOCTL.
> > > 
> > > Prior to this, all commands that the driver exposed were explicitly
> > > enabled. After this, only those commands that are found in the CEL are
> > > enabled.
> > > 
> > > Signed-off-by: Ben Widawsky <ben.widawsky@...el.com>  
> > 
> > This patch made me wonder if the model for the command in quite right.
> > I think it would end up simpler with a pair of payload pointers for send
> > and receive (that can be equal when it makes sense).
> > 
> > A few other things inline.
> > 
> > Jonathan  
> 
> I'll address the others separately, but could you elaborate on this? I'm not
> sure I follow your meaning.

Further down in the review..
"
The fact that you end up bypassing the payload transfer stuff in mbox_cmd
rather suggests it's not a particularly good model.  + it keeps confusing
me.

While the hardware uses a single region for the payload, there is nothing
saying the code has to work that way.   Why not have separate payload_in and
payload_out pointers?  Occasionally you might set them to the same buffer, but
elsewhere you could avoid the direct memcpy()s you are doing around the
send_cmd(). 

"

Jonathan


> 
> [snip]
> 
>