linux-kernel - Re: [PATCH 2/3] fbdev/deferred-io: Support contiguous kernel memory framebuffers

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Message-ID: <20250409105058.GB5572@lst.de>
Date: Wed, 9 Apr 2025 12:50:58 +0200
From: Christoph Hellwig <hch@....de>
To: mhklinux@...look.com
Cc: jayalk@...works.biz, simona@...ll.ch, deller@....de,
	haiyangz@...rosoft.com, kys@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, akpm@...ux-foundation.org, weh@...rosoft.com,
	tzimmermann@...e.de, hch@....de, dri-devel@...ts.freedesktop.org,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hyperv@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/3] fbdev/deferred-io: Support contiguous kernel
 memory framebuffers

On Tue, Apr 08, 2025 at 11:36:45AM -0700, mhkelley58@...il.com wrote:
> In any case, for x86 and arm64 today, commit 37b4837959cb9 is not
> sufficient to support contiguous kernel memory framebuffers. The problem
> can be seen with the hyperv_fb driver, which may allocate the framebuffer
> memory using vmalloc() or alloc_pages(), depending on the configuration
> of the Hyper-V guest VM (Gen 1 vs. Gen 2) and the size of the framebuffer.

And neither is this code.  You need to provide the functionality at
the DMA layer as users must not poke into the returned DMA coherent
memory.