Re: [PATCH] mfd: intel-lpss: use devm_ioremap_uc for mmio

[Mika Westerberg]

Hi,

In $subject, use MMIO instead of mmio.

On Fri, Sep 27, 2019 at 11:55:13AM -0600, Tuowen Zhao wrote:
> Write-combining BAR for intel-lpss-pci in MTRR causes system hangs
> during boot.
> 
> This patch adds devm_ioremap_uc as a new managed wrapper to ioremap_uc
> and with it forces the use of strongly uncachable mmio in intel-lpss.
> 
> This bahavior is seen on Dell XPS 13 7390 2-in-1:
> 
> [    0.001734]   5 base 4000000000 mask 6000000000 write-combining

I think it is worth mentioning that this is actually a BIOS bug and
fixed by some vendors via BIOS upgrade.

> 4000000000-7fffffffff : PCI Bus 0000:00
>   4000000000-400fffffff : 0000:00:02.0 (i915)
>   4010000000-4010000fff : 0000:00:15.0 (intel-lpss-pci)
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=203485
> Signed-off-by: Tuowen Zhao <ztuowen@xxxxxxxxx>
> ---
>  drivers/mfd/intel-lpss.c |  2 +-
>  include/linux/io.h       |  2 ++
>  lib/devres.c             | 19 +++++++++++++++++++
>  3 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/mfd/intel-lpss.c b/drivers/mfd/intel-lpss.c
> index 277f48f1cc1c..06106c9320bb 100644
> --- a/drivers/mfd/intel-lpss.c
> +++ b/drivers/mfd/intel-lpss.c
> @@ -395,7 +395,7 @@ int intel_lpss_probe(struct device *dev,
>  	if (!lpss)
>  		return -ENOMEM;
>  
> -	lpss->priv = devm_ioremap(dev, info->mem->start + LPSS_PRIV_OFFSET,
> +	lpss->priv = devm_ioremap_uc(dev, info->mem->start + LPSS_PRIV_OFFSET,
>  				  LPSS_PRIV_SIZE);

Can you add a comment here explaining why this particular driver needs
to use _uc version? Normally drivers call the plain devm_ioremap() and
expect to get non-cached memory, however ioremap() (which resolves to
ioremap_nocache()) does not touch x86 PAT configuration which is the
reason we need to call _uc variant here.

Otherwise looks good to me.