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

[Luis R. Rodriguez]

On Sat, Mar 10, 2018 at 11:03 AM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
> On Sat, Mar 10, 2018 at 8:57 AM, French, Nicholas A. <naf@xxxxxx> 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);

And  I wonder if its better to have a generic kconfig option so that
in case other drivers have similar issue they can make use of it as
well. For now that's a non-issue, but worth pointing out if we're
going to do this for more than one driver later.

 Luis