linux-kernel - Re: [PATCH] dma-mapping: Stub out dma_{alloc,free,map}

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Message-ID: <20250617155514.GC1376515@ziepe.ca>
Date: Tue, 17 Jun 2025 12:55:14 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Arnd Bergmann <arnd@...db.de>
Cc: Christoph Hellwig <hch@....de>, James Clark <james.clark@...aro.org>,
	Mark Brown <broonie@...nel.org>,
	Vladimir Oltean <olteanv@...il.com>, oe-kbuild-all@...ts.linux.dev,
	Larisa Grigore <larisa.grigore@....com>,
	Frank Li <Frank.li@....com>, linux-spi@...r.kernel.org,
	imx@...ts.linux.dev, linux-kernel@...r.kernel.org,
	kernel test robot <lkp@...el.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev
Subject: Re: [PATCH] dma-mapping: Stub out dma_{alloc,free,map}_pages() API

On Tue, Jun 17, 2025 at 10:26:51AM +0200, Arnd Bergmann wrote:
> On Tue, Jun 17, 2025, at 09:53, Arnd Bergmann wrote:
> 
> > Between SH72xx/SH76xx, SUN3 and M68328, I believe the
> > supported machines are all limited to between 1MB and 32MB in
> > the maximum configuration, which is obviously extremely
> > tight.
> 
> I checked the exact numbers we're talking about here: enabling
> CONFIG_HAS_DMA on rsk7269_defconfig adds 10KB of extra vmlinux
> size, which doesn't seem too bad:
> 
>    text	   data	    bss	    dec	    hex	filename
> 3295084	1111396	 112264	4518744	 44f358	vmlinux-before
> 3302836	1113652	 112264	4528752	 451a70	vmlinux-after

Long ago I ran some numbers for an ancient PPC system:

https://lore.kernel.org/all/20121119214922.GA5636@obsidianresearch.com/

The base smallest kernel was growing .text and a stripped down initrd
at a rate of 1MB evey 6 years.

Somehow I doubt that system (with 16MB ram I think it was) would even
fit a v6.x kernel. v3.6 was already challenging.

Even back then Greg was incredulous that an embedded system would run
a 6 year newer kernel. Here we are contemplating a 20 year newer
kernel?

I think you have the right direction, we just removed !SMP support,
removing !DMA also seems logical to me.

Jason