linux-kernel - Re: ivtv: use arch_phys_wc_add() and require PAT disabled

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Date:   Thu, 8 Mar 2018 04:06:01 +0000
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     "French, Nicholas A." <naf@...edu>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        "hans.verkuil@...co.com" <hans.verkuil@...co.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: ivtv: use arch_phys_wc_add() and require PAT disabled

On Thu, Mar 08, 2018 at 03:16:29AM +0000, French, Nicholas A. wrote:
> On Wed, Mar 07, 2018 at 07:02:05PM +0000, Luis R. Rodriguez wrote:
> > On Tue, Mar 06, 2018 at 09:01:10PM +0000, French, Nicholas A. wrote:
> > > any reason why PAT can't be enabled for ivtvfb as simply as in the attached
> > > patch?
> >
> > Prior to your change the OSD buffer was obtained using the itv->dec_mem + oi->video_rbase
> > given itv->dec_mem was initialized via [...]
> >         itv->dec_mem = ioremap_nocache(itv->base_addr + IVTV_DECODER_OFFSET - oi->video_buffer_size,
> >                                 IVTV_DECODER_SIZE);
> 
> Ah, I see. So my proposed ioremap_wc call was only "working" by aliasing the
> ioremap_nocache()'d mem area and not actually using write combining at all.

There are some debugging PAT toys out there I think but I haven't played with
them yet or I forgot how to to confirm or deny this sort of effort, but
likeley.

> > So what I'd do is change the ioremap_nocache()'d size by substracting
> > oi->video_buffer_size -- but then you have to ask yourself how you'd get
> > that size. If its something you can figure out then great.
> 
> Size is easy since its hardcoded, but unfortunately getting the offset of the
> framebuffer inside the decoders memory to remove from the ioremap_nocache
> call is a chicken and egg problem: the offset is determined by querying the
> firmware that has been loaded to the decoder. the firmware itself will be
> loaded after the ioremap_nocache call at an offset from the address it
> returns.

What I expected. Probably why no one had tried before to clean it up.

> So unless there is a io-re-remap to change the caching status of a subset of
> the decoder's memory once we find out what the framebuffer offset is inside
> the original iremap_nocache'd area, then its a no go for write combining to
> the framebuffer with PAT.

No what if the framebuffer driver is just requested as a secondary step
after firmware loading?

> 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 requirement in
> general. Apparently the throngs of people using this super-popular driver
> 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.

Nope, best you add a feature to just let you disable wc stuff, to enable
life with PAT.

  Luis