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Date:   Wed, 13 Dec 2023 14:51:11 +0000
From:   Alexandru Elisei <alexandru.elisei@arm.com>
To:     Rob Herring <robh@kernel.org>
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Subject: Re: [PATCH RFC v2 11/27] arm64: mte: Reserve tag storage memory
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Hi,

On Wed, Dec 13, 2023 at 08:06:44AM -0600, Rob Herring wrote:
> On Wed, Dec 13, 2023 at 7:05 AM Alexandru Elisei
> <alexandru.elisei@arm.com> wrote:
> >
> > Hi Rob,
> >
> > On Tue, Dec 12, 2023 at 12:44:06PM -0600, Rob Herring wrote:
> > > On Tue, Dec 12, 2023 at 10:38 AM Alexandru Elisei
> > > <alexandru.elisei@arm.com> wrote:
> > > >
> > > > Hi Rob,
> > > >
> > > > Thank you so much for the feedback, I'm not very familiar with device tree,
> > > > and any comments are very useful.
> > > >
> > > > On Mon, Dec 11, 2023 at 11:29:40AM -0600, Rob Herring wrote:
> > > > > On Sun, Nov 19, 2023 at 10:59 AM Alexandru Elisei
> > > > > <alexandru.elisei@arm.com> wrote:
> > > > > >
> > > > > > Allow the kernel to get the size and location of the MTE tag storage
> > > > > > regions from the DTB. This memory is marked as reserved for now.
> > > > > >
> > > > > > The DTB node for the tag storage region is defined as:
> > > > > >
> > > > > >         tags0: tag-storage@8f8000000 {
> > > > > >                 compatible = "arm,mte-tag-storage";
> > > > > >                 reg = <0x08 0xf8000000 0x00 0x4000000>;
> > > > > >                 block-size = <0x1000>;
> > > > > >                 memory = <&memory0>;    // Associated tagged memory node
> > > > > >         };
> > > > >
> > > > > I skimmed thru the discussion some. If this memory range is within
> > > > > main RAM, then it definitely belongs in /reserved-memory.
> > > >
> > > > Ok, will do that.
> > > >
> > > > If you don't mind, why do you say that it definitely belongs in
> > > > reserved-memory? I'm not trying to argue otherwise, I'm curious about the
> > > > motivation.
> > >
> > > Simply so that /memory nodes describe all possible memory and
> > > /reserved-memory is just adding restrictions. It's also because
> > > /reserved-memory is what gets handled early, and we don't need
> > > multiple things to handle early.
> > >
> > > > Tag storage is not DMA and can live anywhere in memory.
> > >
> > > Then why put it in DT at all? The only reason CMA is there is to set
> > > the size. It's not even clear to me we need CMA in DT either. The
> > > reasoning long ago was the kernel didn't do a good job of moving and
> > > reclaiming contiguous space, but that's supposed to be better now (and
> > > most h/w figured out they need IOMMUs).
> > >
> > > But for tag storage you know the size as it is a function of the
> > > memory size, right? After all, you are validating the size is correct.
> > > I guess there is still the aspect of whether you want enable MTE or
> > > not which could be done in a variety of ways.
> >
> > Oh, sorry, my bad, I should have been clearer about this. I don't want to
> > put it in the DT as a "linux,cma" node. But I want it to be managed by CMA.
> 
> Yes, I understand, but my point remains. Why do you need this in DT?
> If the location doesn't matter and you can calculate the size from the
> memory size, what else is there to add to the DT?

I am afraid there has been a misunderstanding. What do you mean by
"location doesn't matter"?

At the very least, Linux needs to know the address and size of a memory
region to use it. The series is about using the tag storage memory for
data. Tag storage cannot be described as a regular memory node because it
cannot be tagged (and normal memory can).

Then there's the matter of the tag storage block size (explained in this
commit message), and also knowing the memory range for which a tag storage
region stores the tags. This is explained in the cover letter.

Is there something that you feel that is not clear enough? I am more than
happy to go into details.

Thanks,
Alex