linux-kernel - Re: ivtv: use arch_phys_wc_add() and require PAT disabled

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Date:   Sat, 10 Mar 2018 10:20:23 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "French, Nicholas A." <naf@...edu>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "hans.verkuil@...co.com" <hans.verkuil@...co.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: ivtv: use arch_phys_wc_add() and require PAT disabled




> On Mar 10, 2018, at 8:57 AM, French, Nicholas A. <naf@...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."
> :-)
> 
>> 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?
> 
> 

None from me. 

However, since you have the hardware, you could see if you can use the change_page_attr machinery to change the memory type on the framebuffer once you figure out where it is.