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:	Fri, 4 Jan 2013 14:34:28 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Shuah Khan <shuahkhan@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Eric W. Biederman" <ebiederm@...ssion.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,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	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 Fri, Jan 4, 2013 at 2:26 PM, Shuah Khan <shuahkhan@...il.com> wrote:

> However, I think regression on existing behavior with a
> panic is a bit of a big hammer. Thie change causes panic on systems
> even when kdump is not enabled, if I understand it correctly.

I don't think so.

+static bool __init enough_mem_for_swiotlb(void)
+{
+       /* do we have less than 1M RAM under 4G ? */
+       return memblock_mem_size(1ULL<<(32-PAGE_SHIFT)) > (1ULL<<20);
+}

enough_mem_for_swiotlb could return false for them?

and

 int __init pci_swiotlb_detect_override(void)
 {
-       int use_swiotlb = swiotlb | swiotlb_force;
-
        if (swiotlb_force)
                swiotlb = 1;
+       else if (!enough_mem_for_swiotlb())
+               swiotlb = 0;

-       return use_swiotlb;
+       return swiotlb;
 }

it only disable swiotlb when there is less 1M mem under 4G.
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