linux-kernel - Re: [PATCH] drm/msm/a6xx: Add support for an interconnect path

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Date:   Tue, 12 Feb 2019 16:07:35 +0200
From:   Georgi Djakov <georgi.djakov@...aro.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     jcrouse@...eaurora.org, robdclark@...il.com, evgreen@...omium.org,
        freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/a6xx: Add support for an interconnect path

Hi Greg,

On 2/12/19 12:16, Greg KH wrote:
> On Tue, Feb 12, 2019 at 11:52:38AM +0200, Georgi Djakov wrote:
>> From: Jordan Crouse <jcrouse@...eaurora.org>
>>
>> Try to get the interconnect path for the GPU and vote for the maximum
>> bandwidth to support all frequencies. This is needed for performance.
>> Later we will want to scale the bandwidth based on the frequency to
>> also optimize for power but that will require some device tree
>> infrastructure that does not yet exist.
>>
>> v6: use icc_set_bw() instead of icc_set()
>> v5: Remove hardcoded interconnect name and just use the default
>> v4: Don't use a port string at all to skip the need for names in the DT
>> v3: Use macros and change port string per Georgi Djakov
>>
>> Signed-off-by: Jordan Crouse <jcrouse@...eaurora.org>
>> Acked-by: Rob Clark <robdclark@...il.com>
>> Reviewed-by: Evan Green <evgreen@...omium.org>
>> Signed-off-by: Georgi Djakov <georgi.djakov@...aro.org>
>> ---
>>
>> Hi Greg,
>>
>> If not too late, could you please take this patch into char-misc-next.
>> It is adding the first consumer of the interconnect API. We are just
>> getting the code in place, without making it functional yet, as some
>> DT bits are still needed to actually enable it. We have Rob's Ack to
>> merge this together with the interconnect code. This patch has already
>> spent some time in linux-next without any issues.
> 
> I have a question about the interconnect code.  Last week I saw a
> presentation about the resctrl/RDT code from ARM that is coming (MPAM),
> and it really looks like the same functionality as this interconnect
> code.  In fact, this code looks like the existing resctrl stuff, right?

Thanks for the question! It's nice that MPAM is moving forward. When i
looked into the MPAM draft spec an year ago, it was an optional
extension mentioning mostly use-cases with VMs on server systems.

But anyway, MPAM is only available for ARMv8.2+ cores as an optional
extension and aarch32 is not supported. In contrast to that, the
interconnect code is generic and does not put any limitations on the
platform/architecture that can use it - just the platform specific
implementation would be different. We have discussed in that past that
it can be used even on x86 platforms to provide hints to firmware.

> So why shouldn't we just drop the interconnect code and use resctrl
> instead as it's already merged?

I haven't seen any MPAM code so far, but i assume that we can have an
interconnect provider that implements this MPAM extension for systems
that support it (and want to use it). Currently there are people working
on various interconnect platform drivers from 5 different SoC vendors
and we have agreed to use a common DT bindings (and API). I doubt that
even a single one of these platforms is based on v8.2+. Probably such
SoCs would be coming in the future and then i expect people making use
of MPAM in some interconnect provider driver.

Thanks,
Georgi