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

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Message-ID: <ZoY9HZwon3_yiq6F@infradead.org>
Date: Wed, 3 Jul 2024 23:11:41 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Icenowy Zheng <uwu@...nowy.me>
Cc: Bjorn Helgaas <helgaas@...nel.org>,
	Jiaxun Yang <jiaxun.yang@...goat.com>,
	Christian König <christian.koenig@....com>,
	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 Thu, Jul 04, 2024 at 10:00:52AM +0800, Icenowy Zheng wrote:
> So I here want to ask a question as an individual hacker: what's the
> policy of linux-pci towards these non-coherent PCIe implementations?
> 
> If the sentences of Christian is right, these implementations are just
> out-of-spec, should them get purged out of the kernel, or at least
> raising a warning that some HW won't work because of inconformant
> implementation?

Nothing in the PCIe specifications that mandates a programming model.
Non-coherent DMA is extremely common in lower end devices, and despite
all the issues that it causes well supported in Linux.

What are you trying to solve?