linux-kernel - Re: [PATCH 08/10] drm/tegra: Implement dynamic channel allocation model

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Message-ID: <ce92a134-a18f-1337-f7f6-031f46ce6fcd@gmail.com>
Date:   Mon, 13 Nov 2017 14:49:00 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Mikko Perttunen <cyndis@...si.fi>,
        Mikko Perttunen <mperttunen@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com
Cc:     dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/10] drm/tegra: Implement dynamic channel allocation
 model

On 07.11.2017 15:29, Mikko Perttunen wrote:
> On 05.11.2017 19:43, Dmitry Osipenko wrote:
>> On 05.11.2017 14:01, Mikko Perttunen wrote:
>>> In the traditional channel allocation model, a single hardware channel
>>> was allocated for each client. This is simple from an implementation
>>> perspective but prevents use of hardware scheduling.
>>>
>>> This patch implements a channel allocation model where when a user
>>> submits a job for a context, a hardware channel is allocated for
>>> that context. The same channel is kept for as long as there are
>>> incomplete jobs for that context. This way we can use hardware
>>> scheduling and channel isolation between userspace processes, but
>>> also prevent idling contexts from taking up hardware resources.
>>>
>>
>> The dynamic channels resources (pushbuf) allocation is very expensive,
>> neglecting all benefits that this model should bring at least in non-IOMMU case.
>> We could have statically preallocated channels resources or defer resources
>> freeing.
> 
> This is true. I'll try to figure out a nice way to keep the pushbuf allocations.

One variant could be to have all channels resources statically preallocated in a
non-IOMMU case because CMA is expensive. Then you should measure the allocations
impact in a case of IOMMU allocations and if it is significant, maybe implement
Host1x PM autosuspend, releasing all channels when Host1x become idle.

I think the above should be efficient and easy to implement.