Re: [PATCH v11 20/27] iommu/exynos: allow having multiple System MMUs for a master H/W

From: Tomasz Figa
Date: Wed Mar 19 2014 - 09:20:52 EST

On 19.03.2014 01:39, Cho KyongHo wrote:
On Tue, 18 Mar 2014 15:26:48 +0100, Tomasz Figa wrote:

On 18.03.2014 14:01, Cho KyongHo wrote:
On Fri, 14 Mar 2014 17:12:03 +0100, Tomasz Figa wrote:
Hi KyongHo,

On 14.03.2014 06:10, Cho KyongHo wrote:
Some master device descriptor like fimc-is which is an abstraction
of very complex H/W may have multiple System MMUs. For those devices,
the design of the link between System MMU and its master H/W is needed
to be reconsidered.

A link structure, sysmmu_list_data is introduced that provides a link
to master H/W and that has a pointer to the device descriptor of a
System MMU. Given a device descriptor of a master H/W, it is possible
to traverse all System MMUs that must be controlled along with the
master H/W.


A device driver should handle particular hardware instances separately,
without abstracting a virtual hardware instance consisting of multiple
physical ones.

If such abstraction is needed, it should be done above the exynos-iommu
driver, e.g. by something like iommu-composite driver that would
aggregate several IOMMUs. Keep in mind that such IOMMUs in a group could
be different, e.g. different Exynos SysMMU versions or even completely
different IPs handled by different drivers.

Still, I don't think there is a real need for such abstraction. Instead,
related drivers shall be fixed to properly handle multiple memory
masters and their IOMMUs.

G2D, Scalers and FIMD of Exynos5420 has 2 System MMUs while aother SoC like
Exynos5250 does not.

I don't understand why you are negative to this approach.
This is the simplest than the others.

Let me show you an example.
FIMC-IS driver just controls MCU in FIMC-IS subsystem and the firmware of
the MCU controls all other peripherals in the subsystem. Each peripherals
have their own System MMU. Moreover, the configuration of the peripherals
varies according to the SoCs.

If System MMU driver accepts multiple masters, everything is done in DT.
But I worry that it is not easy if System MMU driver does not support
multiple masters.

I believe I have stated enough reasons why this kind of implementation
is bad. I'm not going to waste time repeating myself.

Your concerns presented above are valid, however they are not related to
what is wrong with this patch. I have given you two proper ways to
handle this, none should be forced upon particular IOMMU master drivers
- their authors should have the chance to select the method that works
best for them.

I don't still understand why you think this patch is wrong.
I think this is the best way not to think for all the driver developers
about other things than their business logic.

I agree, but one of the ways I proposed (an iommu-composite layer above the IOMMU low level drivers) doesn't add any extra responsibility of driver developers.

Moreover, it's this kind of business logic in low level drivers that is adding more responsibility, because it introduces additional complexity and makes the driver harder to read, maintain and extend in future.

This does not hurt anyone and I think this is good enough.

Well, it is barely good enough. It is a good practice to make a low level driver handle a single device instance and this is how Linux driver model is designed.

Moreover, a single device tree node _must_ represent a single hardware block, so you can't group multiple SysMMUs into a single device tree node.

Furthermore, if you force grouping of SysMMUs into a single virtual one, you enforce using the same address space for all masters of some particular hardware blocks, while potentially driver developers would like to separate them.

A good interface design should not enforce any particular implementation and this is what we should stick to in this case as well.

If you want to provide another layer between master device and system mmu
as you mentioned, you do that. This patch does not restrict it.

It does, as mentioned above. With a device tree listing multiple SysMMUs as one, you can't separate them.

Best regards,
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