Re: [PATCH v3 1/2] omap3: iovmm: Work around sg_alloc_table size limitation in IOMMU

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Fri, Jun 03, 2011 at 02:12:47AM +0200, Laurent Pinchart wrote:
> Hi Russell,
> 
> On Wednesday 01 June 2011 16:03:06 Russell King - ARM Linux wrote:
> > On Wed, Jun 01, 2011 at 03:50:50PM +0200, Laurent Pinchart wrote:
> > > In the specific iovmm case, the driver uses the sglist API to build a
> > > list of page-size sg entries, and then process it in software. Is that
> > > considered as an abuse of the sglist API, or valid usage ?
> > > 
> > > Anyway, sglist chaining is not needed by iovmm. As iovmm just walks the
> > > sglist manually, it's easier to allocate it in one go rather than using
> > > sglist chaining. This of course doesn't make your patch unneeded or
> > > wrong.
> > 
> > Well, there's a two issues here:
> > 1. Should iovmm use sg_phys(sg) with sg_dma_len(sg) ?
> >    Probably not, because a scatterlist before DMA API mapping is defined
> >    by sg_page(sg), sg->offset, sg->length and has N entries.  After DMA
> >    API mapping (n = dma_map_sg(dev, sg, N, dir)), it has n entries where
> >    n <= N, and the DMA address/lengths are sg_dma_address(sg) and
> >    sg_dma_len(sg).  Both these are undefined for unmapped scatterlists.
> > 
> >    Getting this wrong means breakage when CONFIG_NEED_SG_DMA_LENGTH is
> >    enabled.
> 
> iovmm abuses the sglist API, there's no doubt on that. It will break when 
> CONFIG_NEED_SG_DMA_LENGTH is enabled. iovmm should probably not use the sglist 
> API, and it should probably not even exist in the first place. I know that TI 
> is working on moving the OMAP-specific iommu/iovmm implementation to the 
> generic IOMMU API, but that will take time. In the meantime, I'd like to fix 
> iovmm to avoid the userspace-triggerable BUG_ON().
> 
> > 2. What would be the effect of enabling SG list chaining on iovmm?
> >    The code uses the correct SG list walking helpers (for_each_sg) so
> >    it should be able to cope with chained SG lists.
> 
> Yes it should. It might be slightly less efficient, but I don't think we will 
> notice.
> 
> > So, I think there's no problem here with chained SG lists, but there is
> > an issue with using sg_dma_len().  I'd suggest converting stuff to use
> > sg->length with sg_page(sg) rather than sg_dma_len(sg).
> 
> With sg_page(sg) ? I'm not sure to follow you there.

sg->length and sg_page(sg) are paired (and sg->length is paired with
other stuff).  They describe the scatterlist _before_ DMA API mapping.
After DMA API mapping, the scatterlist describes a list of regions
defined by sg_dma_address(sg) and sg_dma_len(sg) - sg_dma_len(sg) is
_only_ paired with sg_dma_address(sg).

> > As for whether SG chaining is required or not, if you're running up against
> > the maximum SG table size, then you do have a requirement for SG chaining.
> 
> The SG table size limit makes sure that the SG list fits in a page, so that it 
> can be passed to the hardware. This isn't needed by iovmm, as it processes the 
> sglist in software. iovmm could use SG chaining, but we would then need to 
> enable it for the SoCs on which iovmm is used. I don't know if they properly 
> support that.

Err, no.  scatterlists are _never_ passed to hardware.  They're a kernel
internal description of a list of regions in memory, which initially
start off as describing the kernels view of those regions.  After DMA
mapping, they describe it in terms of the device's view of those
regions.

At that point, scatterlists get converted to whatever form is required
by the hardware doing DMA, which most certainly won't be the layout which
struct scatterlist describes.

SG chaining has _nothing_ to do with hardware.  It's all to do with
software and hitting the SG table limit.