linux-kernel - Re: [PATCH V8 03/10] PCI: Create PCI library functions in support of DOE mailboxes.

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Date:   Wed, 1 Jun 2022 13:17:38 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Ben Widawsky <ben.widawsky@...el.com>,
        linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH V8 03/10] PCI: Create PCI library functions in support of
 DOE mailboxes.

On Wed, Jun 01, 2022 at 07:56:47PM +0200, Lukas Wunner wrote:
> On Wed, Jun 01, 2022 at 10:16:15AM -0700, Ira Weiny wrote:
> > On Wed, Jun 01, 2022 at 09:18:08AM +0200, Lukas Wunner wrote:
> > > You only need to re-check the Data Object Ready bit on the last-but-one
> > > dword in case the function was reset concurrently.  Per sec. 6.30.2,
> > > "An FLR to a Function must result in the aborting of any DOE transfer
> > > in progress."
> > 
> > I think I disagree.  Even if we do that and an FLR comes before the last read
> > the last read could be 0's.
> 
> PCIe r6.0, Table 7-316 says:
> 
>   "If there is no additional data object ready for transfer, the
>    DOE instance must clear this bit after the entire data object has been
>    transferred, as indicated by software writing to the DOE Read Data
>    Mailbox Register after reading the final DW of the data object."
> 
> Remember that you *read* a dword from the mailbox and then acknowledge
> reception to the mailbox by *writing* a dword to the mailbox.
> 
> So you check that the Data Object Ready bit is set before acknowledging
> the final dword with a register write.  That's race-free.

Ok.

> 
> (I realize me talking about the "last-but-one dword" above was quite
> unclear, sorry about that.)
> 

Ah yes.  Ok, I'll put in a check before the final write.

Ira