linux-kernel - Re: [RESEND v4 1/6] misc: Add Synopsys DesignWare xData IP driver

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Message-ID: <20210209185246.GA494880@bjorn-Precision-5520>
Date:   Tue, 9 Feb 2021 12:52:46 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Gustavo Pimentel <Gustavo.Pimentel@...opsys.com>
Cc:     Krzysztof Wilczyński <kw@...ux.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Derek Kiernan <derek.kiernan@...inx.com>,
        Dragan Cvetic <dragan.cvetic@...inx.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [RESEND v4 1/6] misc: Add Synopsys DesignWare xData IP driver

On Tue, Feb 09, 2021 at 03:28:16PM +0000, Gustavo Pimentel wrote:
> On Mon, Feb 8, 2021 at 22:53:54, Krzysztof Wilczyński <kw@...ux.com> 
> wrote:
> > [...]
> > > Thanks for your review. I will wait for a couple of days, before sending 
> > > a new version of this patch series based on your feedback.
> > 
> > Thank you!
> > 
> > There might be one more change, and improvement, to be done as per
> > Bjorn's feedback, see:
> > 
> >   https://urldefense.com/v3/__https://lore.kernel.org/linux-pci/20210208193516.GA406304@bjorn-Precision-5520/__;!!A4F2R9G_pg!Oxp56pU_UN6M2BhfNRSdYqsFUncqVklBj_1IdLQD_w_V6dKRPDO_FjPUystMa5D39SRj8uo$ 
> > 
> > The code in question would be (exceprt from the patch):
> > 
> > [...]
> > +static int dw_xdata_pcie_probe(struct pci_dev *pdev,
> > +			       const struct pci_device_id *pid)
> > +{
> > +	const struct dw_xdata_pcie_data *pdata = (void *)pid->driver_data;
> > +	struct dw_xdata *dw;
> > [...]
> > +	dw->rg_region.vaddr = pcim_iomap_table(pdev)[pdata->rg_bar];
> > +	if (!dw->rg_region.vaddr)
> > +		return -ENOMEM;
> > [...]
> > 
> > Perhaps something like the following would would?
> > 
> > void __iomem * const *iomap_table;
> > 
> > iomap_table = pcim_iomap_table(pdev);
> > if (!iomap_table)
> >         return -ENOMEM;
> > 
> > dw->rg_region.vaddr = iomap_table[pdata->rg_bar];
> > if (!dw->rg_region.vaddr)
> > 	return -ENOMEM;
> > 
> > With sensible error messages added, of course.  What do you think?
> 
> I think all the improvements are welcome. I will do that.
> My only doubt is if Bjorn recommends removing the 
> iomap_table[pdata->rg_bar] check, after adding the verification on the 
> pcim_iomap_table, because all other drivers doesn't do that.

I misunderstood the usage of pcim_iomap_table() -- it looks like one
must call pcim_iomap_regions() *first*, and test its result, and
*that* is where we should catch any pcim_iomap_table() failures, e.g.,

  rc = pcim_iomap_regions()   # or pcim_iomap_regions_request_all()
  if (rc)
    return rc;                # pcim_iomap_table() or other failure

  vaddr = pcim_iomap_table()[BAR];
  if (!vaddr)
    return -ENOMEM;           # BAR doesn't exist

You *do* correctly call pcim_iomap_regions() first, which calls
pcim_imap_table() internally, so if pcim_iomap_table() were to return
NULL, you should catch it there.

Then we assume that the subsequent "pcim_iomap_table()[BAR]" call will
succeed and NOT return NULL, so it should be safe to index into the
table.  And if the table[BAR] entry is NULL, it means the BAR doesn't
exist or isn't mapped.

That sort of makes sense, but the API design doesn't quite seem
obviously correct to me.  The table was created by
pcim_iomap_regions(), and pcim_iomap_table() is basically retrieving
that artifact.

I wonder if it could be improved by making pcim_iomap_table() strictly
internal to devres.c and having the pcim_iomap functions return the
table directly.  Then the code would look something like this:

  table = pcim_iomap_regions();
  if (IS_ERR(table))
    return PTR_ERR(table);    # pcim_iomap_table() or other failure

  vaddr = table[BAR];         # "table" is guaranteed to be non-NULL
  if (!vaddr)
    return -ENOMEM;

Obviously this is not something you should do for *this* series.
I think you should follow the example of other drivers, which means
keeping your patch exactly as you posted it.  I'm just interested in
opinions on this as a possible future API improvement.

Bjorn