linux-kernel - Re: ioremap_uc() followed by set_memory_wc()

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Date:	Wed, 15 Apr 2015 19:59:47 -0400
From:	Andy Walls <awalls@...metrocast.net>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Toshi Kani <toshi.kani@...com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Roland Dreier <roland@...estorage.com>,
	Juergen Gross <jgross@...e.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Borislav Petkov <bp@...e.de>, Mel Gorman <mgorman@...e.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Davidlohr Bueso <dbueso@...e.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ville Syrjälä <syrjala@....fi>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	linux-media@...r.kernel.org, X86 ML <x86@...nel.org>
Subject: Re: ioremap_uc() followed by set_memory_wc() - burrying MTRR

On Wed, 2015-04-15 at 16:42 -0700, Andy Lutomirski wrote:
> On Wed, Apr 15, 2015 at 3:38 PM, Andy Walls <awalls@...metrocast.net> wrote:

> >
> 
> IMO the right solution would be to avoid ioremapping the whole bar at
> startup.  Instead ioremap pieces once the driver learns what they are.
> This wouldn't have any of these problems -- you'd ioremap() register
> regions and you'd ioremap_wc() the framebuffer once you find it.  If
> there are regions of unknown purpose, just don't map them all.
> 
> Would this be feasible?

Feasible? Maybe.

Worth the time and effort for end-of-life, convential PCI hardware so I
can have an optimally performing X display on a Standard Def Analog TV
screen?   Nope. I don't have that level of nostalgia.

We sort of know where some things are in the MMIO space due to
experimentation and past efforts examining the firmware binary.

Documentation/video4linux/cx2341x/fw-*.txt documents some things.  The
driver code actually codifies a little bit more knowledge.

The driver code for doing transfers between host and card is complex and
fragile with some streams that use DMA, other streams that use PIO,
digging VBI data straight out of card memory, and scatter-gather being
broken on newer firmwares.  Playing around with ioremapping will be hard
to get right and likely cause something in the code to break for the
primary use case of the ivtv supported cards. 

Regards,
Andy

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