linux-kernel - Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it

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Date:	Mon, 7 Jan 2013 19:01:02 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Shuah Khan <shuahkhan@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Jan Kiszka <jan.kiszka@....de>,
	Jason Wessel <jason.wessel@...driver.com>,
	linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not
 enough ram for it

On Mon, Jan 7, 2013 at 6:22 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Yinghai I sat down and read your patch and the approach you are taking
> is totally wrong.

Thanks for check the patch, did you check v3?

>
> The problem is that swiotlb_init() in lib/swiotlb.c does not know how to
> fail without panic'ing the system.

I did not put panic in swiotlb, now I put panic in amd_iommu ops init
when it need extra
swiotlb for unhandled devices by AMD IOMMU.

>
> Which leaves two valid approaches.
> - Create a variant of swiotlb_init that can fail for use on x86 and
>   handle the failure.
> - Delay initializing the swiotlb until someone actually needs a mapping
>   from it.
>
> Delaying the initialization of the swiotlb is out because the code
> needs an early memory allocation to get a large chunk of contiguous
> memory for the bounce buffers.

ok.

>
> Which means the panics that occurr in swiotlb_init() need to be delayed
> until someone something actually needs bounce buffers from the swiotlb.
>
> Although arguably what should actually happen instead of panic() is that
> swiotlb_map_single should simply fail early when it was not possible to
> preallocate bounce buffers.

do you mean: actually needed dma buffer is much less than swiotlb
buffer aka 64M?

Yinghai
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