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Date:   Mon, 1 Feb 2021 14:45:00 -0800 (PST)
From:   David Rientjes <rientjes@google.com>
To:     Ben Widawsky <ben.widawsky@intel.com>
cc:     linux-cxl@vger.kernel.org, linux-acpi@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-pci@vger.kernel.org, Bjorn Helgaas <helgaas@kernel.org>,
        Chris Browy <cbrowy@avery-design.com>,
        Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Jon Masters <jcm@jonmasters.org>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Vishal Verma <vishal.l.verma@intel.com>,
        daniel.lll@alibaba-inc.com,
        "John Groves (jgroves)" <jgroves@micron.com>,
        "Kelley, Sean V" <sean.v.kelley@intel.com>
Subject: Re: [PATCH 03/14] cxl/mem: Find device capabilities
In-Reply-To: <20210201223314.qh24uxd7ajdppgfl@intel.com>
Message-ID: <f86149f8-3aea-9d8c-caa9-62771bf22cb5@google.com>
References: <20210130002438.1872527-1-ben.widawsky@intel.com> <20210130002438.1872527-4-ben.widawsky@intel.com> <234711bf-c03f-9aca-e0b5-ca677add3ea@google.com> <20210201165352.wi7tzpnd4ymxlms4@intel.com> <32f33dd-97a-8b1c-d488-e5198a3d7748@google.com>
 <20210201215857.ud5cpg7hbxj2j5bx@intel.com> <b46ed01-3f1-6643-d371-7764c3bde4f8@google.com> <20210201222859.lzw3gvxuqebukvr6@intel.com> <20210201223314.qh24uxd7ajdppgfl@intel.com>
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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?
> > 
> > So it's probably a spec bug, but actually the payload size is 21 bits... I'll
> > check if that was a mistake.
> 
> Well I guess they wanted to be able to specify 1M exactly... Spec should
> probably say you can't go over 1M
> 

I think that's what 8.2.8.4.3 says, no?  And then 8.2.8.4.5 says you 
can use up to Payload Size.  That's why my recommendation was to enforce 
this in cxl_mem_setup_mailbox() up front.