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

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Date:   Thu, 14 Jan 2021 11:04:25 -0800
From:   Ben Widawsky <ben.widawsky@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.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 21-01-14 18:32:17, Jonathan Cameron wrote:
> 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
> 
> 

Ah I was confused if that was a separate statement.

Can you specify the function prototype you're hoping for (or modification to the
structure)?

I really like the lowest level function to simply model the hardware. I get to
write the 8 steps out and clearly implement them.

I personally don't think it's so awkward, but again, give me something more
specific and I'll consider it.