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

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Date:	Wed, 15 Apr 2015 13:42:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>, linux-rdma@...r.kernel.org
Cc:	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>,
	Andy Walls <awalls@...metrocast.net>,
	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 Mon, Apr 13, 2015 at 10:49 AM, Luis R. Rodriguez <mcgrof@...e.com> wrote:

> c) ivtv: the driver does not have the PCI space mapped out separately, and
> in fact it actually does not do the math for the framebuffer, instead it lets
> the device's own CPU do that and assume where its at, see
> ivtvfb_get_framebuffer() and CX2341X_OSD_GET_FRAMEBUFFER, it has a get
> but not a setter. Its not clear if the firmware would make a split easy.
> We'd need ioremap_ucminus() here too and __arch_phys_wc_add().
>

IMO this should be conceptually easy to split.  Once we get the
framebuffer address, just unmap it (or don't prematurely map it) and
then ioremap the thing.

> From the beginning it seems only framebuffer devices used MTRR/WC, lately it
> seems infiniband drivers also find good use for for it for PIO TX buffers to
> blast some sort of data, in the future I would not be surprised if other
> devices found use for it.

IMO the Infiniband maintainers should fix their code.  Especially in
the server space, there aren't that many MTRRs to go around.  I wrote
arch_phys_wc_add in the first place because my server *ran out of
MTRRs*.

Hey, IB people: can you fix your drivers to use arch_phys_wc_add
(which is permitted to be a no-op) along with ioremap_wc?  Your users
will thank you.

> It may be true that the existing drivers that
> requires the above type of work are corner cases -- but I wouldn't hold my
> breath for that. The ivtv device is a good example of the worst type of
> situations and these days. So perhap __arch_phys_wc_add() and a
> ioremap_ucminus() might be something more than transient unless hardware folks
> get a good memo or already know how to just Do The Right Thing (TM).

I disagree.  We should try to NACK any new code that can't function
without MTRRs.

(Plus, ARM is growing in popularity in the server space, and ARM quite
sensibly doesn't have MTRRs.)

--Andy
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