linux-kernel - Re: [PATCH v1] riscv: make ZONE

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Message-ID: <ZwN-E279o4bUQG26@infradead.org>
Date: Sun, 6 Oct 2024 23:22:11 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Vladimir Kondratiev <Vladimir.Kondratiev@...ileye.com>
Cc: Christoph Hellwig <hch@...radead.org>, Alexandre Ghiti <alex@...ti.fr>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH v1] riscv: make ZONE_DMA32 optional

On Mon, Oct 07, 2024 at 06:17:14AM +0000, Vladimir Kondratiev wrote:
> >Well, this doesn't get any more true by just irgnoring the previous
> >discussion and just reposting :(
> 
> Sorry, this wasn't the intention. Perhaps I messed with the message-id, I see my "patch v1" in one mail thread together with the previous discussion, but not in the other thread.
> 
> Anyway, I think making ZONE_DMA32 selection depend on NONPORTABLE answers the concern
> that was raised

It doesn't at all.

For one not having ZONE_DMA32 is going to break a lot of things.
Drivers do expect 32-bit addressable memory.  And because SOC designers
know this there usually is a way to provide it, e.g. by doing window
translations between cpu physical and bus physical address.  Please go
back to your data sheet or talk to the designers.

And if there really is not way to provide this, the right way is just
to stop the runtime allocation that triggered you to do this if
ZONE_DMA32 is empty, not to add a non-portable option.