linux-kernel - Re: [PATCH v2 2/2] serial: 8250_lpss: Switch to pcim_iomap() instead of pci_ioremap

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Date:   Wed, 23 Feb 2022 13:37:18 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] serial: 8250_lpss: Switch to pcim_iomap() instead
 of pci_ioremap_bar()

On Wed, Feb 23, 2022 at 12:13:07PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Feb 23, 2022 at 01:08:07PM +0200, Andy Shevchenko wrote:
> > On Tue, Feb 22, 2022 at 08:02:10AM -0800, Christoph Hellwig wrote:
> > > On Tue, Feb 22, 2022 at 10:14:16AM +0100, Jiri Slaby wrote:
> > > > On 16. 02. 22, 9:53, Christoph Hellwig wrote:
> > > > > On Tue, Feb 15, 2022 at 03:43:59PM +0200, Andy Shevchenko wrote:
> > > > > > The pci_iounmap() doesn't cover all the cases where resource should
> > > > > > be unmapped. Instead of spreading it more, replace the pci_ioremap_bar()
> > > > > > with pcim_iomap() which uses managed resource approach.
> > > > > 
> > > > > pcim_iomap requires the use of ioreadX/iowriteX and thus runtime
> > > > > overhead.  So in doubt please add a pcim_ioremap_bar instead of forcing
> > > > > the legacy iomap/ioread/iowrite API onto modern drivers tht can't
> > > > > support legacy port I/O.
> > > > 
> > > > Hmm, the driver combines pci_ioremap_bar with pci_iounmap. pci_iounmap does
> > > > the right thing after all, but is that correct? And this driver is not
> > > > alone, this shows more:
> > > > git grep -E 'pci_iounmap|pci_ioremap_bar' `git grep -l pci_iounmap \`git
> > > > grep -l pci_ioremap_bar\``
> > > 
> > > I think it is wrong.  It is not actively harmful unlike the the
> > > combination of pci_iomap and then later use of accessors from the
> > > ioremap family, but still not exactly a good idea.
> > > 
> > > In a perfect world we'd have some different annotation from __iomem
> > > for the whole iomap family of functions.
> > 
> > So, what would be your suggestion for a) backportable change b) cleanup for
> > the current and future drivers?
> 
> Worry about getting it right first.  Only after that should you even
> consider stable tree backports.  There's usually no reason you can't
> just take the same change there as well.  And if not, we will work
> through it :)

Okay, so if I read this thread correctly Christoph suggests to introduce
pcim_ioremap_bar() and then use it. Am I right?

Christoph, since we are on the topic about pcim_*() APIs, can you chime in
the discussion [1] about IRQ vectors allocation?

[1]: https://lore.kernel.org/all/20210607153916.1021016-1-zhengdejin5@gmail.com/

-- 
With Best Regards,
Andy Shevchenko