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List:       linux-pci
Subject:    Re: [PATCH] arm: cns3xxx: fix writing to wrong PCI registers after alignment
From:       Bjorn Helgaas <helgaas () kernel ! org>
Date:       2018-12-31 15:29:18
Message-ID: 20181231152918.GE159477 () google ! com
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On Mon, Dec 31, 2018 at 11:14:28AM +0100, Krzysztof Hałasa wrote:
> Bjorn Helgaas <helgaas@kernel.org> writes:
> 
> > 802b7c06adc7 replaced cns3xxx_pci_write_config(), which always used
> > __raw_writel() so it only did 32-bit accesses, with
> > pci_generic_config_write(), which uses writeb/writew/writel()
> > depending on the size.
> >
> > 802b7c06adc7 also converted cns3xxx_pci_read_config() from always
> > using __raw_readl() (a 32-bit access) to using
> > pci_generic_config_read32(), which also always does a 32-bit access.
> >
> > This makes me think the cnx3xxx hardware is only capable of 32-bit
> > accesses, and this patch should change the driver to use
> > pci_generic_config_write32() instead of pci_generic_config_write() in
> > addition to the mapping fix above.
> 
> Hasn't it already been verified that the CNS3xxx can do 8-bit accesses
> (and probably 16-bit ones as well), and that the docs don't mention any
> such limitation?

Quite possibly.  I'm not familiar with the hardware or docs so can't
comment personally.

802b7c06adc7 reimplemented cns3xxx_pci_read_config() using
pci_generic_config_read32(), which preserved the property of only
doing 32-bit reads.  It replaced cns3xxx_pci_write_config() with
pci_generic_config_write(), so it changed writes from always being 32
bits to being the actual size.  I think this was an error in the
original commit, since the changelog doesn't mention this change; in
fact, the changelog says it changes __raw_writel to writel.

The change from 32-bit to actual size accesses would logically be a
separate commit from changing to use the generic accessors.

Assuming the hardware does support smaller accesses, I'd propose two
patches:

  1) The current one that corrects the address alignment error, and
  2) A new one that converts cns3xxx_pci_read_config() to use
     pci_generic_config_read() instead of pci_generic_config_read32().

Ideally the changelog for the second patch would refer to the
verification that the hardware is capable of 8- and 16-bit accesses.

Bjorn
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