Re: [PATCH 0/9] arm64: tegra: Prevent early SMMU faults

From: Dmitry Osipenko <digetx@gmail.com>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>,
	iommu@lists.linux-foundation.org,
	Jon Hunter <jonathanh@nvidia.com>,
	Nicolin Chen <nicolinc@nvidia.com>,
	linux-tegra@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 0/9] arm64: tegra: Prevent early SMMU faults
Date: Fri, 26 Mar 2021 19:55:55 +0300	[thread overview]
Message-ID: <9f1a8455-f7a6-1f66-f36c-032abacf71f5@gmail.com> (raw)
In-Reply-To: <YF4NQPee+/Qi6zVd@orome.fritz.box>

26.03.2021 19:35, Thierry Reding пишет:
> On Fri, Mar 26, 2021 at 06:29:28PM +0300, Dmitry Osipenko wrote:
>> 25.03.2021 16:03, Thierry Reding пишет:
>>> From: Thierry Reding <treding@nvidia.com>
>>>
>>> Hi,
>>>
>>> this is a set of patches that is the result of earlier discussions
>>> regarding early identity mappings that are needed to avoid SMMU faults
>>> during early boot.
>>>
>>> The goal here is to avoid early identity mappings altogether and instead
>>> postpone the need for the identity mappings to when devices are attached
>>> to the SMMU. This works by making the SMMU driver coordinate with the
>>> memory controller driver on when to start enforcing SMMU translations.
>>> This makes Tegra behave in a more standard way and pushes the code to
>>> deal with the Tegra-specific programming into the NVIDIA SMMU
>>> implementation.
>>
>> It is an interesting idea which inspired me to try to apply a somewhat similar thing to Tegra SMMU driver by holding the SMMU ASID enable-bit until display driver allows to toggle it. This means that we will need an extra small tegra-specific SMMU API function, but it should be okay.
>>
>> I typed a patch and seems it's working good, I'll prepare a proper patch if you like it.
> 
> That would actually be working around the problem that this patch was
> supposed to prepare for. The reason for this current patch series is to
> make sure SMMU translation isn't enabled until a device has actually
> been attached to the SMMU. Once it has been attached, the assumption is
> that any identity mappings will have been created.
> 
> One Tegra SMMU that shouldn't be a problem because translations aren't
> enabled until device attach time. So in other words this patch set is to
> get Tegra186 and later to parity with earlier chips from this point of
> view.
> 
> I think the problem that you're trying to work around is better solved
> by establishing these identity mappings. I do have patches to implement
> this for Tegra210 and earlier, though they may require additional work
> if you have bootloaders that don't use standard DT bindings for passing
> information about the framebuffer to the kernel.

I'm not sure what else reasonable could be done without upgrading to a
very specific version of firmware, which definitely isn't a variant for
older devices which have a wild variety of bootloaders, customized
use-cases and etc.

We could add a kludge that I'm suggesting as a universal fallback
solution, it should work well for all cases that I care about.

So we could have the variant with identity mappings, and if mapping
isn't provided, then fall back to the kludge.