linux-kernel - Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

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Date:   Fri, 1 May 2020 11:37:12 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Jeremy Linton <jeremy.linton@....com>, linux-usb@...r.kernel.org,
        stern@...land.harvard.edu, git@...gavinli.com,
        jarkko.sakkinen@...ux.intel.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] usb: usbfs: correct kernel->user page attribute mismatch

On Fri, May 01, 2020 at 09:05:00AM +0200, Greg KH wrote:
> On Thu, Apr 30, 2020 at 04:19:22PM -0500, Jeremy Linton wrote:
> > On arm64, and possibly other architectures, requesting
> > IO coherent memory may return Normal-NC if the underlying
> > hardware isn't coherent. If these pages are then
> > remapped into userspace as Normal, that defeats the
> > purpose of getting Normal-NC, as well as resulting in
> > mappings with differing cache attributes.
> 
> What is "Normal-NC"?

Arm terminology for "Normal Non-Cacheable"; it might be better to say
something like:

On some architectures (e.g. arm64) an IO coherent mapping may use
non-cachable attributes if the relevant device is cache coherent.
If userspace mappings are cacheable, these may not be coherent with
non-cacheable mappings. On arm64 this is the case for Normal-NC and
Normal (cacheable) mappings.

Thanks,
Mark.