On 4/11/2018 8:03 AM, Robin Murphy wrote:
> On 10/04/18 21:59, Sinan Kaya wrote:
>> Code is expecing to observe the same number of buffers returned from
>> dma_map_sg() function compared to sg_alloc_table_from_pages(). This
>> doesn't hold true universally especially for systems with IOMMU.
> 
> So why not fix said code? It's clearly not a real hardware limitation, and 
> the map_sg() APIs have potentially returned fewer than nents since forever, 
> so there's really no excuse.

Sure, I'll take a better fix if there is one.

> 
>> IOMMU driver tries to combine buffers into a single DMA address as much
>> as it can. The right thing is to tell the DMA layer how much combining
>> IOMMU can do.
> 
> Disagree; this is a dodgy hack, since you'll now end up passing scatterlists 
> into dma_map_sg() which already violate max_seg_size to begin with, and I 
> think a conscientious DMA API implementation would be at rights to fail the 
> mapping for that reason (I know arm64 happens not to, but that was a 
> deliberate design decision to make my life easier at the time).
> 
> As a short-term fix, at least do something like what i915 does and constrain 
> the table allocation to the desired segment size as well, so things remain 
> self-consistent. But still never claim that faking a hardware constraint as a 
> workaround for a driver shortcoming is "the right thing to do" ;)

You are asking for something like this from here, right?

https://elixir.bootlin.com/linux/v4.16.1/source/drivers/gpu/drm/i915/i915_gem_dmabuf.c#L58


        ret = sg_alloc_table(st, obj->mm.pages->nents, GFP_KERNEL);
        if (ret)
                goto err_free;

        src = obj->mm.pages->sgl;
        dst = st->sgl;
        for (i = 0; i < obj->mm.pages->nents; i++) {
                sg_set_page(dst, sg_page(src), src->length, 0);
                dst = sg_next(dst);
                src = sg_next(src);
        }

This seems to allocate the scatter gather list and fill it in manually before 
passing it
to dma_map_sg(). I'll give it a try. 

Just double checking.

> 
> Robin.
> 
>> Signed-off-by: Sinan Kaya <ok...@codeaurora.org>


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux 
Foundation Collaborative Project.
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