linux-kernel - Re: [PATCH 03/14] cxl/mem: Find device capabilities

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Message-ID: <b46ed01-3f1-6643-d371-7764c3bde4f8@google.com>
Date:   Mon, 1 Feb 2021 14:23:47 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Ben Widawsky <ben.widawsky@...el.com>
cc:     linux-cxl@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-pci@...r.kernel.org, Bjorn Helgaas <helgaas@...nel.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jon Masters <jcm@...masters.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        daniel.lll@...baba-inc.com,
        "John Groves (jgroves)" <jgroves@...ron.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>
Subject: Re: [PATCH 03/14] cxl/mem: Find device capabilities

On Mon, 1 Feb 2021, Ben Widawsky wrote:

> > > > > +static int cxl_mem_setup_mailbox(struct cxl_mem *cxlm)
> > > > > +{
> > > > > +	const int cap = cxl_read_mbox_reg32(cxlm, CXLDEV_MB_CAPS_OFFSET);
> > > > > +
> > > > > +	cxlm->mbox.payload_size =
> > > > > +		1 << CXL_GET_FIELD(cap, CXLDEV_MB_CAP_PAYLOAD_SIZE);
> > > > > +
> > > > > +	/* 8.2.8.4.3 */
> > > > > +	if (cxlm->mbox.payload_size < 256) {
> > > > > +		dev_err(&cxlm->pdev->dev, "Mailbox is too small (%zub)",
> > > > > +			cxlm->mbox.payload_size);
> > > > > +		return -ENXIO;
> > > > > +	}
> > > > 
> > > > Any reason not to check cxlm->mbox.payload_size > (1 << 20) as well and 
> > > > return ENXIO if true?
> > > 
> > > If some crazy vendor wanted to ship a mailbox larger than 1M, why should the
> > > driver not allow it?
> > > 
> > 
> > Because the spec disallows it :)
> 
> I don't see it being the driver's responsibility to enforce spec correctness
> though. In certain cases, I need to use the spec, like I have to pick /some/
> mailbox timeout. For other cases... 
> 
> I'm not too familiar with what other similar drivers may or may not do in
> situations like this. The current 256 limit is mostly a reflection of that being
> too small to even support advertised mandatory commands. So things can't work in
> that scenario, but things can work if they have a larger register size (so long
> as the BAR advertises enough space).
> 

I don't think things can work above 1MB, either, though.  Section 
8.2.8.4.5 specifies 20 bits to define the payload length, if this is 
larger than cxlm->mbox.payload_size it would venture into the reserved 
bits of the command register.

So is the idea to allow cxl_mem_setup_mailbox() to succeed with a payload 
size > 1MB and then only check 20 bits for the command register?