linux-kernel - Re: [EXT] Re: [PATCH v16 7/7] PCI: endpoint: pci-epf-vntb: fix sparse build warning at ntb->reg

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Date:   Wed, 14 Dec 2022 05:35:12 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Frank Li <frank.li@....com>
Cc:     "mani@...nel.org" <mani@...nel.org>,
        "allenbh@...il.com" <allenbh@...il.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
        "jdmason@...zu.us" <jdmason@...zu.us>,
        "kw@...ux.com" <kw@...ux.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "lpieralisi@...nel.org" <lpieralisi@...nel.org>,
        "ntb@...ts.linux.dev" <ntb@...ts.linux.dev>
Subject: Re: [EXT] Re: [PATCH v16 7/7] PCI: endpoint: pci-epf-vntb: fix
 sparse build warning at ntb->reg

On Wed, Dec 14, 2022 at 12:49:15AM +0000, Frank Li wrote:
> > 
> > On Wed, Nov 02, 2022 at 10:10:14AM -0400, Frank Li wrote:
> > > From: Frank Li <frank.li@....com>
> > >
> > >   pci-epf-vntb.c:1128:33: sparse:     expected void [noderef] __iomem
> > *base
> > >   pci-epf-vntb.c:1128:33: sparse:     got struct epf_ntb_ctrl *reg
> > >
> > > Add __iomem type convert in vntb_epf_peer_spad_read() and
> > > vntb_epf_peer_spad_write().
> > 
> > I don't understand all the bits and pieces here, but I'm a little
> > dubious about adding all these "(void __iomem *)"casts.  There are
> > very few of them in drivers/pci/, and I doubt this driver is so unique
> > that it needs them.
> 
> sparse compiler report warning without cast.  I write it at commit message.

As a matter of fact, I did read your commit message.  My point is that
I don't think littering the code with casts is the best solution.  I
wrote more details below; please read the entire email.

> > > @@ -1121,7 +1121,7 @@ static u32 vntb_epf_spad_read(struct ntb_dev
> > *ndev, int idx)
> > >       struct epf_ntb *ntb = ntb_ndev(ndev);
> > >       int off = ntb->reg->spad_offset, ct = ntb->reg->spad_count *
> > sizeof(u32);
> > >       u32 val;
> > > -     void __iomem *base = ntb->reg;
> > > +     void __iomem *base = (void __iomem *)ntb->reg;
> > >
> > >       val = readl(base + off + ct + idx * sizeof(u32));
> > >       return val;
> > > @@ -1132,7 +1132,7 @@ static int vntb_epf_spad_write(struct ntb_dev
> > *ndev, int idx, u32 val)
> > >       struct epf_ntb *ntb = ntb_ndev(ndev);
> > >       struct epf_ntb_ctrl *ctrl = ntb->reg;
> > >       int off = ctrl->spad_offset, ct = ctrl->spad_count * sizeof(u32);
> > > -     void __iomem *base = ntb->reg;
> > > +     void __iomem *base = (void __iomem *)ntb->reg;
> > >
> > >       writel(val, base + off + ct + idx * sizeof(u32));
> > 
> > These things look gratuitously different to begin with:
> > 
> >   int off = ntb->reg->spad_offset, ct = ntb->reg->spad_count * sizeof(u32);
> >   int off = ctrl->spad_offset, ct = ctrl->spad_count * sizeof(u32);
> > 
> > They're doing the same thing, and they should do it the same way.
> > 
> > Since db_data[] and db_offset[] are never referenced except to be
> > initialized to zero, I'm guessing the point of vntb_epf_spad_read()
> > and vntb_epf_spad_write() is to read/write things in those arrays?
> > 
> > You access other things in ntb->reg directly by dereferencing a
> > pointer, e.g.,
> > 
> >   ntb->reg->link_status |= LINK_STATUS_UP;
> >   addr = ntb->reg->addr;
> >   ctrl->command_status = COMMAND_STATUS_OK;
> > 
> > Why don't you just compute the appropriate *index* and access the
> > array directly instead of using readl() and writel()?
> > 
> > Bjorn