linux-kernel - Re: [PATCH V13 8/9] cxl/port: Retry reading CDAT on failure

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Message-ID: <20220719160711.000078f9@Huawei.com>
Date:   Tue, 19 Jul 2022 16:07:11 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     <ira.weiny@...el.com>, Bjorn Helgaas <bhelgaas@...gle.com>,
        Ben Widawsky <bwidawsk@...nel.org>,
        Alison Schofield <alison.schofield@...el.com>,
        "Lukas Wunner" <lukas@...ner.de>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
        <linux-pci@...r.kernel.org>
Subject: Re: [PATCH V13 8/9] cxl/port: Retry reading CDAT on failure

On Thu, 14 Jul 2022 09:27:04 -0700
Dan Williams <dan.j.williams@...el.com> wrote:

> ira.weiny@ wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > The CDAT read may fail for a number of reasons but mainly it is possible
> > to get different parts of a valid state.  The checksum in the CDAT table
> > protects against this.  
> 
> I don't know what "different parts of a valid state" means. The CDAT
> should not be changing as it is being read unless someone is issuing a
> set-partition while the DOE operation is happening.

Unfortunately not true. The device is allowed to change it with no input
from OS software at all.

>From CDAT spec

"For Revision=1, the following changes are permitted during the
runtime
• Changes to the latency and bandwidth fields in DSLBIS
• Changes to the latency and bandwidth fields in SSLBIS
• Changes to the number of DSEMTS instances and their
contents
The changes to latency and bandwidth may represent events such
as failover or degradation that are internal to a component."

> Rather than
> arbitrary retries, block out set-partition while CDAT is being read.

Blocking that out is still useful even though we probably still need retries.

> 
> You can use {set,clear}_exclusive_cxl_commands() to temporarily lock out
> set-partition while the CDAT read is happening.
> 
> ...and since this series is only for enabling