Re: [RFC] Remove AGP support from Radeon/Nouveau/TTM

From: "Michel Dänzer" <michel@daenzer.net>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: "Christian König" <ckoenig.leichtzumerken@gmail.com>,
	"Maling list - DRI developers" <dri-devel@lists.freedesktop.org>,
	"Rui Salvaterra" <rsalvaterra@gmail.com>,
	"debian-powerpc@lists.debian.org"
	<debian-powerpc@lists.debian.org>,
	"John Paul Adrian Glaubitz" <glaubitz@physik.fu-berlin.de>,
	"Karoly Balogh (Charlie/SGR)" <charlie@scenergy.dfmk.hu>
Subject: Re: [RFC] Remove AGP support from Radeon/Nouveau/TTM
Date: Wed, 13 May 2020 12:32:52 +0200	[thread overview]
Message-ID: <6bdda31c-f1a4-558c-7f60-111c5e733fbd@daenzer.net> (raw)
In-Reply-To: <CAKMK7uGiFOcEdHUNxxRMr1urX7JDFtgSEnc2TzCWG1UJ=ygN2w@mail.gmail.com>

On 2020-05-13 12:29 p.m., Daniel Vetter wrote:
> On Wed, May 13, 2020 at 12:26 PM Michel Dänzer <michel@daenzer.net> wrote:
>>
>> On 2020-05-13 11:28 a.m., Rui Salvaterra wrote:
>>> On Wed, 13 May 2020 at 08:19, Daniel Vetter <daniel@ffwll.ch> wrote:
>>>>
>>>> i915 is even worse, we manually mess around with clflush. In
>>>> userspace. So really there's 2 axis for dma memory: coherent vs.
>>>> non-coherent (which is something the dma-api somewhat exposed), i.e.
>>>> do you need to clflush or not, and cached vs uncached, i.e. are the
>>>> PAT entries wc or wb.
>>>
>>> So, the PowerPC AGP GART ends up being cached and non-coherent, right
>>> (assuming there's no way to set the page attributes MTRR/PAT-style)?
>>
>> It was uncached when I was using my last-gen PowerBook (until a few
>> years ago), though it's possible that broke since then. I don't remember
>> the details how it's done offhand though.
>>
>> The only theoretical problem there was that the kernel still had a
>> cacheable mapping of the same memory, and any access via that (e.g.
>> prefetch due to access to a neighbouring page) could trigger a machine
>> check. But I don't remember ever hitting that. Maybe I was just lucky
>> all those years.
> 
> At least on arm this has been a big topic, since it indeed randomly
> kills machines. That's why you can't remap random pages as wc, they
> have to be in highmem. I thought ppc is equally easily angered. And
> the trouble is that just the existance of the mapping is enough to
> cause a machine check exception iirc. So down to pure luck.

Either myself and others were very lucky then with PowerMacs, or it's
not actually that bad.

-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer
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