Re: ioremap_uc() followed by set_memory_wc() - burrying MTRR

From: Andy Lutomirski
Date: Wed Apr 15 2015 - 20:58:47 EST

On Wed, Apr 15, 2015 at 4:59 PM, Andy Walls <awalls@xxxxxxxxxxxxxxxx> wrote:
> On Wed, 2015-04-15 at 16:42 -0700, Andy Lutomirski wrote:
>> On Wed, Apr 15, 2015 at 3:38 PM, Andy Walls <awalls@xxxxxxxxxxxxxxxx> 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.

The point is actually to let us unexport or delete mtrr_add. We can
either severely regress performance on ivtv on PAT-capable hardware if
we naively switch it to arch_phys_wc_add or we can do something else.
The something else remains to be determined.

> 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.


If the only thing that really wants WC is the esoteric framebuffer
thing, could we just switch to arch_phys_wc_add and assume that no one
will care about the regression on new CPUs with ivtv cards?

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