[lkml]   [2019]   [Dec]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 0/8] Convert the intel iommu driver to the dma-iommu api
On 2019-12-23 10:37 am, Jani Nikula wrote:
> On Sat, 21 Dec 2019, Tom Murphy <> wrote:
>> This patchset converts the intel iommu driver to the dma-iommu api.
>> While converting the driver I exposed a bug in the intel i915 driver
>> which causes a huge amount of artifacts on the screen of my
>> laptop. You can see a picture of it here:
>> This issue is most likely in the i915 driver and is most likely caused
>> by the driver not respecting the return value of the
>> dma_map_ops::map_sg function. You can see the driver ignoring the
>> return value here:
>> Previously this didn’t cause issues because the intel map_sg always
>> returned the same number of elements as the input scatter gather list
>> but with the change to this dma-iommu api this is no longer the
>> case. I wasn’t able to track the bug down to a specific line of code
>> unfortunately.
>> Could someone from the intel team look at this?
> Let me get this straight. There is current API that on success always
> returns the same number of elements as the input scatter gather
> list. You propose to change the API so that this is no longer the case?

No, the API for dma_map_sg() has always been that it may return fewer
DMA segments than nents - see Documentation/DMA-API.txt (and otherwise,
the return value would surely be a simple success/fail condition).
Relying on a particular implementation behaviour has never been strictly
correct, even if it does happen to be a very common behaviour.

> A quick check of various dma_map_sg() calls in the kernel seems to
> indicate checking for 0 for errors and then ignoring the non-zero return
> is a common pattern. Are you sure it's okay to make the change you're
> proposing?

Various code uses tricks like just iterating the mapped list until the
first segment with zero sg_dma_len(). Others may well simply have bugs.


> Anyway, due to the time of year and all, I'd like to ask you to file a
> bug against i915 at [1] so this is not forgotten, and please let's not
> merge the changes before this is resolved.
> Thanks,
> Jani.
> [1]

 \ /
  Last update: 2019-12-23 12:30    [W:0.155 / U:3.804 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site