linux-kernel - Re: [PATCH V10 6/9] cxl/port: Read CDAT table

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Date:   Thu, 9 Jun 2022 15:03:27 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
CC:     Ben Widawsky <bwidawsk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Alison Schofield" <alison.schofield@...el.com>,
        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 V10 6/9] cxl/port: Read CDAT table

On Thu, Jun 09, 2022 at 09:27:38AM +0100, Jonathan Cameron wrote:
> On Wed, 8 Jun 2022 14:27:14 -0700
> Ira Weiny <ira.weiny@...el.com> wrote:
> 
> > On Mon, Jun 06, 2022 at 11:15:41AM -0700, Ben Widawsky wrote:
> > > On 22-06-04 17:50:46, ira.weiny@...el.com wrote:  
> > > > From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > > >   
> > 
> 
> [snip]
> 
> > >   
> > > > +
> > > > +		entry = cdat_response_pl + 1;
> > > > +		entry_dw = task.rv / sizeof(u32);
> > > > +		/* Skip Header */
> > > > +		entry_dw -= 1;
> > > > +		entry_dw = min(length / 4, entry_dw);
> > > > +		memcpy(data, entry, entry_dw * sizeof(u32));
> > > > +		length -= entry_dw * sizeof(u32);
> > > > +		data += entry_dw;
> > > > +		entry_handle = FIELD_GET(CXL_DOE_TABLE_ACCESS_ENTRY_HANDLE, cdat_response_pl[0]);  
> > > 
> > > [0] looks suspicious...  
> > 
> > Actually I have to claim ignorance on this one.  I've carried this from
> > Jonathan's original patches.  I'm not as worried about the [0] as that is just
> > the first dword.  But I'm confused as to this entry handle now.
> > 
> > Jonathan?  Help?
> 
> Looks right to me.  The entryhandle is a field in the upper 16 bits of the
> first dword defined in Read Entry Response table in the CXL spec and also
> used in the request of the next entry (which is more or less a CDAT structure)
> Two magic values.
> 0 -  CDAT header (request only - can't be returned)
> 0xFFFF - No more entries.

Thanks!

> 
> As we are reading the whole table, we write 0 to first request and from there
> on use the value returned in the response for the next request until we see
> 0xFFFF and stop.
> 
> Note IIRC the meaning of entry handle was clarified in a CXL 2.0 errata as
> was a bit ambiguous in the original spec (we had two QEMU implementations
> briefly and they did different things :)

I see it now!  I'm going to add a define for 0xFFFF as well.

Ira

> 
> Jonathan
> 
> 
> > 
> > >   
> > > > +
> > > > +	} while (entry_handle != 0xFFFF);
> > > > +
> > > > +	return rc;
> > > > +}
> > > > +
> 
>