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

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Date:   Tue, 13 Dec 2022 18:26:42 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Frank Li <Frank.Li@....com>
Cc:     mani@...nel.org, allenbh@...il.com, bhelgaas@...gle.com,
        dave.jiang@...el.com, imx@...ts.linux.dev, jdmason@...zu.us,
        kw@...ux.com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, lpieralisi@...nel.org,
        ntb@...ts.linux.dev
Subject: Re: [PATCH v16 7/7] PCI: endpoint: pci-epf-vntb: fix sparse build
 warning at ntb->reg

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.

> @@ -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