Re: [RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Christian Gmeiner <christian.gmeiner@gmail.com>]

2015-02-03 13:28 GMT+01:00 Russell King - ARM Linux <linux@arm.linux.org.uk>:
> On Tue, Feb 03, 2015 at 08:48:56AM +0100, Daniel Vetter wrote:
>> On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote:
>> > On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
>> > >> My initial thought is for dma-buf to not try to prevent something than
>> > >> an exporter can actually do.. I think the scenario you describe could
>> > >> be handled by two sg-lists, if the exporter was clever enough.
>> > >
>> > > That's already needed, each attachment has it's own sg-list. After all
>> > > there's no array of dma_addr_t in the sg tables, so you can't use one sg
>> > > for more than one mapping. And due to different iommu different devices
>> > > can easily end up with different addresses.
>> >
>> >
>> > Well, to be fair it may not be explicitly stated, but currently one
>> > should assume the dma_addr_t's in the dmabuf sglist are bogus.  With
>> > gpu's that implement per-process/context page tables, I'm not really
>> > sure that there is a sane way to actually do anything else..
>>
>> Hm, what does per-process/context page tables have to do here? At least on
>> i915 we have a two levels of page tables:
>> - first level for vm/device isolation, used through dma api
>> - 2nd level for per-gpu-context isolation and context switching, handled
>>   internally.
>>
>> Since atm the dma api doesn't have any context of contexts or different
>> pagetables, I don't see who you could use that at all.
>
> What I've found with *my* etnaviv drm implementation (not Christian's - I
> found it impossible to work with Christian, especially with the endless
> "msm doesn't do it that way, so we shouldn't" responses and his attitude
> towards cherry-picking my development work [*]) is that it's much easier to
> keep the GPU MMU local to the GPU and under the control of the DRM MM code,
> rather than attaching the IOMMU to the DMA API and handling it that way.
>

Keep in mind that I tried to reach you several times via mail and irc
and you simply
ignored me. Did you know that took almost all of your patches (with
small changes)?
And I needed to cherry pick you patches as they were a) wrong, b) solved in a
different way or c) had "hack" in the subject. I am quite sorry that I
ended that
way, but it is not only my fault!

> There are several reasons for that:
>
> 1. DRM has a better idea about when the memory needs to be mapped to the
>    GPU, and it can more effectively manage the GPU MMU.
>
> 2. The GPU MMU may have TLBs which can only be flushed via a command in
>    the GPU command stream, so it's fundamentally necessary for the MMU to
>    be managed by the GPU driver so that it knows when (and how) to insert
>    the flushes.
>
>
> * - as a direct result of that, I've stopped all further development of
> etnaviv drm, and I'm intending to strip it out from my Xorg DDX driver
> as the etnaviv drm API which Christian wants is completely incompatible
> with the non-etnaviv drm, and that just creates far too much pain in the
> DDX driver.
>

That is bad, but life moves on.

greets
--
Christian Gmeiner, MSc

https://soundcloud.com/christian-gmeiner

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