linux-kernel - Re: [PATCH v13 6/6] iommu/arm-smmu: add support for PRR bit setup

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Message-ID: <61e04616-32f8-4574-9721-ea032d227bf2@linaro.org>
Date: Tue, 16 Jul 2024 14:14:11 +0200
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Rob Clark <robdclark@...il.com>,
 Bibek Kumar Patro <quic_bibekkum@...cinc.com>
Cc: will@...nel.org, robin.murphy@....com, joro@...tes.org, jgg@...pe.ca,
 jsnitsel@...hat.com, robh@...nel.org, krzysztof.kozlowski@...aro.org,
 quic_c_gdjako@...cinc.com, dmitry.baryshkov@...aro.org,
 iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Rob Clark <robdclark@...omium.org>
Subject: Re: [PATCH v13 6/6] iommu/arm-smmu: add support for PRR bit setup

On 15.07.2024 10:07 PM, Rob Clark wrote:
> On Mon, Jul 15, 2024 at 4:00 AM Bibek Kumar Patro
> <quic_bibekkum@...cinc.com> wrote:

[...]

>>>> As I checked gpu_prr_mem reserved mem section is not used for mobile
>>>> targets hence not present for other DT only compute targets like
>>>> x1e80100.dtsi has the same. PRR looks to be smmu version specific
>>>> property.
>>>
>>> I only see it in gpu_prr_mem in x1e80100.dtsi, but not documented
>>> anywhere.  I'm only assuming based on the name that it is intended to
>>> be for PRR (but not sure why it is larger than 0x1000).  Are the
>>> PRR_CFG_*ADDR regs programmed by the fw (and access blocked in EL1) on
>>> this device?
>>>
>>
>> As I checked, if the drm/gfx driver allocates the page for drm, then
>> this reserved-memory region is not required.
>>
>> PRR_CFG_*ADDR regs have read and write access in EL1 only for this
>> device, behavior is same as other devices as well. These are not
>> programmed by fw.
> 
> If there is any device which _doesn't_ have EL1 access to these regs,
> I think going the reserved memory route seems more future proof?
> Otherwise we later on have to deal with two different ways to do
> things.  But I'm not sure if there is any such device or risk.

We can have our cake and eat it too, if we keep the check for a
reserved memory node handle, but make it a dynamic allocation (see
[1] for example), this way there's a way to opt into using this from
the DT and there's no need for adding more properties

Konrad

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/qcom/msm8916.dtsi?h=v6.10#n109