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From:   "French, Nicholas A." <naf@ou.edu>
To:     "Luis R. Rodriguez" <mcgrof@kernel.org>,
        Andy Lutomirski <luto@kernel.org>
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Subject: Re: ivtv: use arch_phys_wc_add() and require PAT disabled
Thread-Topic: ivtv: use arch_phys_wc_add() and require PAT disabled
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Date:   Thu, 8 Mar 2018 05:23:09 +0000
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On Thu, Mar 08, 2018 at 04:14:11AM +0000, Luis R. Rodriguez wrote:
> On Thu, Mar 08, 2018 at 04:06:01AM +0000, Luis R. Rodriguez wrote:
>  > On Thu, Mar 08, 2018 at 03:16:29AM +0000, French, Nicholas A. wrote:
> > >=20
>  > > Ah, I see. So my proposed ioremap_wc call was only "working" by alia=
sing the
>  > > ioremap_nocache()'d mem area and not actually using write combining =
at all.
>  >=20
>  > There are some debugging PAT toys out there I think but I haven't play=
ed with
>  > them yet or I forgot how to to confirm or deny this sort of effort, bu=
t
>  > likeley.
>=20
>  In fact come to think of it I believe some neurons are telling me that i=
f
>  two type does not match we'd get an error?

I based my guess on some text i read in "PATting Linux" [1]:
"ioremap interfaces will succeed if there is an existing,
more lenient mapping. Example: If there is an existing
uncached mapping to a physical range, any request for
write-back or write-combine mapping will succeed, but
will eventually map the memory as uncached"

But I will try to get some debugpat going to confirm.

[1] =3D https://www.kernel.org/doc/ols/2008/ols2008v2-pages-135-144.pdf

> > So unless there is a io-re-remap to change the caching status of a subs=
et of
> > the decoder's memory once we find out what the framebuffer offset is in=
side
> > the original iremap_nocache'd area, then its a no go for write combinin=
g to
> > the framebuffer with PAT.
>=20
> No what if the framebuffer driver is just requested as a secondary step
> after firmware loading?

Its a possibility. The decoder firmware gets loaded at the beginning of the=
 decoder
memory range and we know its length, so its possible to ioremap_nocache eno=
ugh=20
room for the firmware only on init and then ioremap the remaining non-firmw=
are
decoder memory areas appropriately after the firmware load succeeds...
=20
> > > On the other hand, it works fine for me with a nocache'd framebuffer.=
 It's
> > > certainly better for me personally to have a nocache framebuffer with
> > > PAT-enabled than the framebuffer completely disabled with PAT-enabled=
, but I
> > > don't think I would even propose to rollback the x86 nopat requiremen=
t in
> > > general. Apparently the throngs of people using this super-popular dr=
iver
> > > feature haven't complained in the last couple years, so maybe its OK =
for me
> > > to just patch the pat-enabled guard out and deal with a nocache'd
> > > framebuffer.
> >=20
> > Nope, best you add a feature to just let you disable wc stuff, to enabl=
e
> > life with PAT.

I'm not sure I understand what you mean.

Perhaps the easy answer is to change the fatal is-pat-enabled check to just=
 a
warning like "you have PAT enabled, so wc is disabled for the framebuffer.=
=20
if you want wc, use the nopat parameter"?

- Nick=