Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is notenough ram for it

From: Yinghai Lu
Date: Mon Jan 07 2013 - 22:00:55 EST

On Mon, Jan 7, 2013 at 6:22 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> 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.


> 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?

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