linux-kernel - Re: [PATCH v2] dma-contiguous: define proper name for global cma region

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Date:   Thu, 3 Aug 2023 23:04:02 +0530
From:   Pintu Agarwal <pintu.ping@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     John Stultz <jstultz@...gle.com>,
        Pintu Kumar <quic_pintu@...cinc.com>,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, m.szyprowski@...sung.com, robin.murphy@....com,
        iommu@...ts.linux.dev, Sumit Semwal <sumit.semwal@...aro.org>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        Liam Mark <lmark@...eaurora.org>,
        Laura Abbott <labbott@...hat.com>,
        Brian Starkey <Brian.Starkey@....com>,
        Christian König <christian.koenig@....com>,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v2] dma-contiguous: define proper name for global cma region

Hi,

On Wed, 2 Aug 2023 at 15:17, Christoph Hellwig <hch@....de> wrote:
>
> On Tue, Aug 01, 2023 at 10:39:04PM -0700, John Stultz wrote:
> > So, forgive me, I've not had a chance to look into this, but my
> > recollection was "reserved" is the name we see on x86, but other names
> > are possibly provided via the dts node?
>
No, I think "reserved" is the name hard-coded (for all arch) in Kernel
for global-cma.
So, I don't think this is x86 specific. I am checking on arm32 itself.
When we can dma_alloc_coherent we see these in the logs (if dts region
is not present).
cma: cma_alloc(cma (ptrval), name: reserved, count 64, align 6)
Now, with this change we will see this:
cma: cma_alloc(cma (ptrval), name: global-cma-region, count 64, align 6)

> Indeed, dma_contiguous_default_area can also be set through
> rmem_cma_setup, which then takes the name from DT.
>
I think this is a different case. If DT entry is present we get this:
Reserved memory: created CMA memory pool at 0x98000000, name: name:
linux,cma, size 128 MiB
cma: cma_alloc(cma (ptrval), name: linux,cma, count 64, align 6)

Here we are talking about the default hard-coded name in Kernel code
if DT is not defined.
So, in one of the boards, this DT entry was not present and it shows
as "reserved".

> > I believe on the hikey board its "linux,cma" is the name, so forcing
> > it to reserved would break that.
> >
Yes, everywhere in the DT it's defined as "linux,cma".
You mean this also should be changed to "linux,cma-global-region"
everywhere with this change ?

> > Maybe instead add a compat config option to force the cma name (so x86
> > can set it to "default" if needed)?
>
Yes, having it in config is also a good option instead of hard-coding in Kernel.
>
> I think we'll just need to leave it as-is.  I with dma-heaps had never
> exposed the name to userspace, but we'll have to lіve with it now.

Can you point me to the userspace utility we are talking about here ?
I think we should not worry much about userspace name exposure.
I guess it should fetch whatever is declared in Kernel or DTS, right ?