linux-kernel - Re: commit 7ffb791423c7 breaks steam game

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Message-ID: <16501a89-65f8-4750-9b99-f8fbcbfe5e3d@nvidia.com>
Date: Tue, 25 Mar 2025 21:25:33 +1100
From: Balbir Singh <balbirs@...dia.com>
To: Christian König <christian.koenig@....com>,
 Bert Karwatzki <spasswolf@....de>
Cc: Ingo Molnar <mingo@...nel.org>, Kees Cook <kees@...nel.org>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>,
 Alex Deucher <alexander.deucher@....com>, linux-kernel@...r.kernel.org,
 amd-gfx@...ts.freedesktop.org
Subject: Re: commit 7ffb791423c7 breaks steam game

On 3/25/25 19:36, Christian König wrote:
> Am 25.03.25 um 00:07 schrieb Bert Karwatzki:
>> Here's the dmesg from linux-next-6.14-rc7-next20250321 (CONFIG_PCI_P2PDMA not set)
>> The memory ranges of (afe00000000-affffffffff) or (3ffe00000000-3fffffffffff) are
>> mentioned in neither of them.
> 
> Ugh, next time either in two mails or as attachment please. I had to separate the logs manually again.
> 
> The BIOS programs multiple root PCIe resources:
> [    0.310962] [      T1] pci_bus 0000:00: root bus resource [mem 0x000a0000-0x000dffff window]
> [    0.310963] [      T1] pci_bus 0000:00: root bus resource [mem 0xf0000000-0xfcffffff window]
> [    0.310965] [      T1] pci_bus 0000:00: root bus resource [mem 0x1010000000-0xffffffffff window]
> 
> But by default all BARs end up in the 0xf0000000-0xfcffffff window and are so accessible to the iGPU.
> 
> So I don't see a reason why dma_addressing_limited() should return true.
> 

Because of

	if (min_not_zero(dma_get_mask(dev), dev->bus_dma_limit) <
			 dma_get_required_mask(dev))

IIRC, dma_get_required_mask is set to 46 bits (based on previous logs) for the iGPU

"[    3.965669] [    T328] dma_addressing_limited: mask = 0xfffffffffff bus_dma_limit = 0x0 required_mask = 0x3fffffffffff
[    3.965671] [    T328] dma_addressing_limited: returning true
[    3.965672] [    T328] amdgpu 0000:08:00.0: amdgpu: amdgpu_ttm_init: calling ttm_device_init() with use_dma32 = 1"

I am not sure why that is the case at the moment

Balbir