linux-kernel - Re: [PATCH] x86: Don't make AMD_GART depend on EXPERT and default y

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Message-ID: <20131006094227.GC4535@gmail.com>
Date:	Sun, 6 Oct 2013 11:42:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>, bp@...e.de
Subject: Re: [PATCH] x86: Don't make AMD_GART depend on EXPERT and default y


* Andi Kleen <andi@...stfloor.org> wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> The AMD_GART driver was made EXPERT/EMBEDDED a long time
> ago to avoid unbootable 64bit systems with 32bit only devices.
> 
> This was before swiotlb was there, which does the job
> of this fallback today. SWIOTLB is always on, so systems
> should always boot.
> 
> The drawback is that every system has to compile that
> driver in (it cannot be a module).
> 
> Also:
> - Newer AMD CPUs (the APUs) don't seem to have AMD_GART support
> at all anymore.
> - Newer AMD platforms have a much better real IOMMU
> - The AMD GART driver was never very good (lots of overhead, e.g.
> in flushing due to some workarounds) and it's doubtful it's really
> better than SWIOTLB.
> - On older K8 systems it didn't even work with all chipsets.
> - The 32bit device bounce buffer case should be rare/
> non performance critical these days anyways.
> - On non AMD systems it is not needed at all.
> 
> So drop the EXPERT dependency on AMD_GART and remove the
> default y. The driver can be still compiled in, just
> it's an explicit decision now, and people who don't want
> it can unselect it.
> 
> I also clarified the description a bit.
> 
> This allows to save ~8K text on most modern x86-64 systems.
> 
> Cc: bp@...e.de
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>

Btw., this is a perfect changelog!

Thanks,

	Ingo
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