Re: [PATCH v1 1/2] drm/qxl: replace ioremap by ioremap_cache on arm64

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On Wed, Mar 23, 2022 at 09:45:13AM +0000, Robin Murphy wrote:
> On 2022-03-23 07:15, Christian König wrote:
> > Am 22.03.22 um 10:34 schrieb Cong Liu:
> > > qxl use ioremap to map ram_header and rom, in the arm64 implementation,
> > > the device is mapped as DEVICE_nGnRE, it can not support unaligned
> > > access.
> > 
> > Well that some ARM boards doesn't allow unaligned access to MMIO space
> > is a well known bug of those ARM boards.
> > 
> > So as far as I know this is a hardware bug you are trying to workaround
> > here and I'm not 100% sure that this is correct.
> No, this one's not a bug. The Device memory type used for iomem mappings is
> *architecturally* defined to enforce properties like aligned accesses, no
> speculation, no reordering, etc. If something wants to be treated more like
> RAM than actual MMIO registers, then ioremap_wc() or ioremap_cache() is the
> appropriate thing to do in general (with the former being a bit more
> portable according to Documentation/driver-api/device-io.rst).

Well, qxl is a virtual device, so it *is* ram.

I'm wondering whenever qxl actually works on arm?  As far I know all
virtual display devices with (virtual) pci memory bars for vram do not
work on arm due to the guest mapping vram as io memory and the host
mapping vram as normal ram and the mapping attribute mismatch causes
caching troubles (only noticeable on real arm hardware, not in
emulation).  Did something change here recently?

take care,

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