linux-kernel - Re: PCIe coherency in spec (was: [RFC PATCH 2/2] drm/ttm: downgrade cached to write

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Message-ID: <20240703210831.GA63958@bhelgaas>
Date: Wed, 3 Jul 2024 16:08:31 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Jiaxun Yang <jiaxun.yang@...goat.com>
Cc: Christian König <christian.koenig@....com>,
	Icenowy Zheng <uwu@...nowy.me>, Huang Rui <ray.huang@....com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	bhelgaas@...gle.com, dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org
Subject: Re: PCIe coherency in spec (was: [RFC PATCH 2/2] drm/ttm: downgrade
 cached to write_combined when snooping not available)

On Wed, Jul 03, 2024 at 04:52:30PM +0800, Jiaxun Yang wrote:
> 在2024年7月2日七月 下午6:03，Jiaxun Yang写道：
> > 在2024年7月2日七月 下午5:27，Christian König写道：
> >> Am 02.07.24 um 11:06 schrieb Icenowy Zheng:
> >>> [SNIP] However I don't think the definition of the AGP spec could apply on all
> >>> PCI(e) implementations. The AGP spec itself don't apply on
> >>> implementations that do not implement AGP (which is the most PCI(e)
> >>> implementations today), and it's not in the reference list of the PCIe
> >>> spec, so it does no help on this context. 
> >> No, exactly that is not correct.
> >>
> >> See as I explained the No-Snoop extension to PCIe was created to help 
> >> with AGP support and later merged into the base PCIe specification.
> >>
> >> So the AGP spec is now part of the PCIe spec.
> 
> Hi Bjorn & linux-pci folks,
> 
> It seems like we have some disputes on interpretation pf PCIe
> specification.
> 
> We are seeking your expertise on the question: Does PCIe
> specification mandate Cache coherency via snoop?

I'm not qualified to opine on this.  I'd say it's a question for the
PCI SIG protocol workgroup.  https://forum.pcisig.com/ is a place to
start.

Bjorn