linux-kernel - Re: [RFC PATCH 2/3] pci/doe: Use devm_xa

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Message-ID: <YshR9b+cc1VogVlc@iweiny-desk3>
Date:   Fri, 8 Jul 2022 08:49:09 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Bjorn Helgaas <helgaas@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
        <linux-pci@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/3] pci/doe: Use devm_xa_init()

On Fri, Jul 08, 2022 at 04:04:05PM +0100, Matthew Wilcox wrote:
> On Fri, Jul 08, 2022 at 07:57:10AM -0700, Ira Weiny wrote:
> > > > I'll update this to be more clear in a V1 if it goes that far.  But to clarify
> > > > here; the protocol information is a u16 vendor id and u8 protocol number.  So
> > > > we are able to store that in the unsigned long value that would normally be a
> > > > pointer to something in the XArray.
> > > 
> > > Er.  Signed long.
> > 
> > Sorry I misspoke, xa_mk_value() takes an unsigned long.
> 
> It does, *but* ...
> 
> static inline void *xa_mk_value(unsigned long v)
> {
>         WARN_ON((long)v < 0);
>         return (void *)((v << 1) | 1);
> }
> 
> ... you can't pass an integer that has the top bit set to it.
> 
> > Can't I use xa_mk_value() to store data directly in the entry "pointer"?
> 
> Yes, that's the purpose of xa_mk_value().  From what you said, it sounded
> like you were just storing the integer directly, which won't work.
> 
> > +static void *pci_doe_xa_prot_entry(u16 vid, u8 prot)
> > +{
> > +	return xa_mk_value(((unsigned long)vid << 16) | prot);
> > +}
> > 
> > Both Dan and I thought this was acceptable in XArray?
> 
> You haven't tested that on 32-bit, have you?  Shift vid by 8 instead of
> 16, and it'll be fine.

Ah ok.

> 
> (Oh, and you don't need to cast vid; the standard C integer promotions
> will promote vid to int before shifting, and you won't lose any bits)

Will do, thanks!
Ira