Messages in this thread |  | | From | (Eric W. Biederman) | Date | Mon, 07 Jan 2013 19:13:46 -0800 | Subject | Re: [PATCH v7u1 26/31] x86: Don't enable swiotlb if there is not enough ram for it |
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Yinghai Lu <yinghai@kernel.org> writes:
> On Mon, Jan 7, 2013 at 6:22 PM, Eric W. Biederman <ebiederm@xmission.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?
I looked at the version of the patch you had as an attachment. I don't know the version number but it was the latest version of the patch I saw in this thread.
After looking at things having a function enoung_mem_for_swiotlb() in pci_swiotlb_detect_override() and pic_swiotlb_detect_4gb is brittle hack.
>> 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.
But the only reason you need to touch this code at all is that swiotlb_init() calls panic() if you don't have 64MB of memory below 4G.
>> 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?
I meant we should detect failure to allocate bounce buffers in in swiotlb_init() instead of panicing.
I meant swiotlb_map_single() should either panic or simply fail.
If I have read lib/swiotlb.c correctly the only place we allocate a bounce buffer is in swiotlb_map_single. If there are more places we can allocate bounce buffers those need to be handled as well.
Eric
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