linux-kernel - Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous

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Message-ID: <CAA+hA=RnEh8QG1vHSFBwMF2pwxv-J14m3za3yaqcNgqb=+xezA@mail.gmail.com>
Date:   Wed, 19 May 2021 11:54:56 +0800
From:   Dong Aisheng <dongas86@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Dong Aisheng <aisheng.dong@....com>,
        iommu@...ts.linux-foundation.org,
        open list <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/1] dma-contiguous: return early for dt case in dma_contiguous_reserve

On Wed, May 19, 2021 at 2:51 AM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2021-05-18 12:28, Dong Aisheng wrote:
> > dma_contiguous_reserve() aims to support cmdline case for CMA memory
> > reserve. But if users define reserved memory in DT,
> > 'dma_contiguous_default_area' will not be 0, then it's meaningless
> > to continue to run dma_contiguous_reserve(). So we return early
> > if detect 'dma_contiguous_default_area' is unzero.
>
> But dma_contiguous_default_area *shouldn't* be set if the command-line
> argument is present - see the "if (size_cmdline != -1 && default_cma)"
> part of rmem_cma_setup(). Are you seeing something different in practice?
>

yes, you're right.

> > Cc: Christoph Hellwig <hch@....de>
> > Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> > Cc: Robin Murphy <robin.murphy@....com>
> > Signed-off-by: Dong Aisheng <aisheng.dong@....com>
> > ---
> >   kernel/dma/contiguous.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/dma/contiguous.c b/kernel/dma/contiguous.c
> > index 3d63d91cba5c..ebade9f43eff 100644
> > --- a/kernel/dma/contiguous.c
> > +++ b/kernel/dma/contiguous.c
> > @@ -171,6 +171,9 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
> >       phys_addr_t selected_limit = limit;
> >       bool fixed = false;
> >
> > +     if (dma_contiguous_default_area)
> > +             return;
> > +
> >       pr_debug("%s(limit %08lx)\n", __func__, (unsigned long)limit);
> >
> >       if (size_cmdline != -1) {
> > @@ -191,7 +194,7 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
> >   #endif
> >       }
> >
> > -     if (selected_size && !dma_contiguous_default_area) {
> > +     if (selected_size) {
>
> Either way, does skipping a handful of trivial calculations and a
> debugging message really matter even when it is redundant? I can't
> imagine it has any measurable effect on boot times...
>

I think it's not about performance.
It aims to improve the code readability as it's meaningless to continue to
execute cmdline CMA reserve logic once DT is used successfully which is
a bit confusing when people first read this part of code.
Does it make sense to you?

Regards
Aisheng

> Robin.
>
> >               pr_debug("%s: reserving %ld MiB for global area\n", __func__,
> >                        (unsigned long)selected_size / SZ_1M);
> >
> >