Re: ivtv: use arch_phys_wc_add() and require PAT disabled

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Sat, Mar 10, 2018 at 8:57 AM, French, Nicholas A. <naf@ou.edu> wrote:
> On Wed, Mar 07, 2018 at 11:23:09PM -0600, French, Nicholas A. wrote:
>> 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:
>> > > >
>> > > > 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.
>> >
>> >  In fact come to think of it I believe some neurons are telling me that if
>> >  two type does not match we'd get an error?
>
> I can confirm that my original suggested patch just aliases to ivtv-driver's nocache mapping:
> $ sudo modprobe ivtvfb
> $ sudo dmesg
> ...
> x86/PAT: Overlap at 0xd5000000-0xd5800000
> x86/PAT: reserve_memtype added [mem 0xd5510000-0xd56b0fff], track uncached-minus, req write-combining, ret uncached-minus
> ivtvfb0: Framebuffer at 0xd5510000, mapped to 0x00000000c6a7ed52, size 1665k
> ...
> $ sudo cat /sys/kernel/debug/x86/pat_memtype_list | grep 0xd5
> uncached-minus @ 0xd5000000-0xd5800000
> uncached-minus @ 0xd5510000-0xd56b1000
>
> So nix that.
>
>> > 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 enough
>> room for the firmware only on init and then ioremap the remaining non-firmware
>> decoder memory areas appropriately after the firmware load succeeds...
>
> I looked in more detail, and this would be "hard" due to the way the rest of the
> decoder offsets are determined by either making firmware calls or scanning the
> decoder memory range for magic bytes and other mess.
>
> I think some smart guy named mcgrof apparently came to the same conclusion
> in a really old email chain I found [https://lists.gt.net/linux/kernel/2387536]:
> "The ivtv case is the *worst* example we can expect where the firmware
> hides from us the exact ranges for write-combining, that we should somehow
> just hope no one will ever do again."
> :-)

This is tribal knowledge worth formalizing a bit more for the long run
for this ivtv driver.

>> 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.
>> if you want wc, use the nopat parameter"?
>
> I like this idea more and more. I haven't experience any problems running
> with PAT-enabled and no write-combining on the framebuffer. Any objections?

I think its worth it, and perhaps best folded under a new kernel
parameter option which also documents the limitation noted above,
thereby knocking two birds with one stone. This way also users who
*want* to opt-in to PAT do so willing-fully and knowing of the
limitation. The kconfig option can just enable a module parameter to a
default value, which if the kconfig is disabled would otherwise be
unset.

static bool ivtv_force_pat = IS_ENABLED(CONFIG_IVTV_WHATEVER);
module_param_named(force_pat, ivtv_force_pat, bool, S_IRUGO | S_IWUSR);

  Luis