On 27/04/18 18:39, Thomas Hellstrom wrote:
On 04/27/2018 06:56 PM, Robin Murphy wrote:
Hi Thomas,
On 25/04/18 14:21, Thomas Hellstrom wrote:
Hi, Robin,
Thanks for the patch. It was some time since I put together that
code, but I remember hitting something similar to
https://urldefense.proofpoint.com/v2/url?u=https-3A__www.linuxquestions.org_questions_linux-2Dkernel-2D70_-2527nents-2527-2Dargument-2Dof-2Ddma-5Funmap-5Fsg-2D4175621964_&d=DwIDaQ&c=uilaK90D4TOVoH58JNXRgQ&r=wnSlgOCqfpNS4d02vP68_E9q2BNMCwfD2OZ_6dCFVQQ&m=UACKfhMfw9wac0BNUWXnAiivjaBgY_jAEupre0zXoOQ&s=8NQNd-XBCViYHJH4fHk-RluFYd9CDjbYzXl_BWhC0ig&e=
Even if it's clear from the documentation that orig_nents should be
used.
Hmmm, it's odd that you would see issues - it's always been something
that CONFIG_DMA_API_DEBUG would have screamed about, and as far as I'm
aware for x86, nents and orig_nents should always end up equal anyway.
I would definitely be interested to see the specific fault details if
it can be reproduced. I suppose one possibility is that there's some
path where you inadvertently unmap something which was never mapped,
but passing nents=0 means you manage to get away with it without the
DMA API backend trying to interpret any bogus DMA addresses/lengths.
FWIW, the rationale is that sync_sg/unmap_sg operate on sg->page
(which can always be translated back to a meaningful CPU address for
cache/write buffer maintenance), not sg->dma_address (which sometimes
cannot), therefore passing a truncated list will have the effect of
just not syncing the tail end of the buffer, which is clearly bad.
Robin.
I agree. I browsed the current software- and hardware iommu dma backends
and all of them seem to set nents == orig_nents.
Still, according to the docs sg_map() is free to erase any sg->page -
related information and given that, it would be more natural if all
operations after sg_map() would operate on sg->dma_address.
TBH I've never really understood that aspect of the API - I've yet to
come across an implementation which does actually destroy the non-DMA
parts of the scatterlist, and I can't think of a good reason why one
would ever need or want to.
I mean if
sg_map would truncate the sg list length to 1, and erase all other
information (which it clearly is free to do according to the docs), it
would be pretty meaningless to supply orig_nents for the unmapping
operation?
Sure, but not all implementations can work that way - a bounce-buffering
scheme, for example, could quite feasibly merge pages in the bounce
buffer and return a single DMA segment, but definitely needs all the
original page info in unmap_sg in order to be able to copy new DMA data
back to the right places. A non-coherent IOMMU implementation *could*
walk the IOMMU page tables for every page worth of the DMA segment to
get back to the relevant CPU addresses to perform cache maintenance
with, but that's horrendously inefficient compared to just using the
original CPU view directly (notably, they *do* have to do such a table
walk for unmap_page, but only once since a single mapping is always
physically contiguous by definition).
Robin.
/Thomas
On 04/13/2018 05:14 PM, Robin Murphy wrote:
dma_unmap_sg() should be called with the same number of entries
originally passed to dma_map_sg(), not the number it returned, which
may
be fewer. Admittedly this driver probably never runs on non-coherent
architectures where getting that wrong could lead to data loss, but
it's
always good to be correct, and it's trivially easy to fix by just
restoring the SG table state before the call instead of afterwards.
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
Found by inspection while poking around TTM users.
drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
b/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
index 21111fd091f9..971223d39469 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c
@@ -369,9 +369,9 @@ static void vmw_ttm_unmap_from_dma(struct
vmw_ttm_tt *vmw_tt)
{
struct device *dev = vmw_tt->dev_priv->dev->dev;
+ vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;
dma_unmap_sg(dev, vmw_tt->sgt.sgl, vmw_tt->sgt.nents,
DMA_BIDIRECTIONAL);
- vmw_tt->sgt.nents = vmw_tt->sgt.orig_nents;
}
/**
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