On 11/09/18 17:30, Wolfram Sang wrote:
Hi all,
commit 78c47830a5cb ("dma-debug: check scatterlist segments") triggers
for Renesas hardware I look after, so thanks for pointing out we should
have proper dma_parms for our DMA providers.
When trying to fix it, I became a bit puzzled about the life cycle of
the pointer to dma_parms. AFAIU most drivers leave the pointer dangling
on driver unbind. Check drivers/dma/bcm2835-dma.c, for example:
od = devm_kzalloc(&pdev->dev, sizeof(*od), GFP_KERNEL);
if (!od)
return -ENOMEM;
pdev->dev.dma_parms = &od->dma_parms;
dma_set_max_seg_size(&pdev->dev, 0x3FFFFFFF);
And that's all about handling dma_parms. So, on unbind, the memory for
'od' gets freed and dma_params is a dangling pointer.
That's the typical case - the dma_parms structure is simply embedded in
some other private data which tends to have the appropriate lifetime
anyway. I can't see that the dangling pointer is an issue when it's set
unconditionally on probe and valid until remove, because anyone
dereferencing dev->dma_parms when dev has no driver bound is doing
something very very wrong anyway. I suppose we could zero it in
__device_release_driver() for good measure though - shame we've found
something dma_deconfigure() could have been useful for just after we
killed it ;)
drivers/gpu/drm/exynos/exynos_drm_iommu.c seems to do it correctly:
...but could be even simpler if it was just included in
exynos_drm_private. Realistically, I struggle to imagine a driver which
would need to set dma_parms that didn't already have some other private
state into which it could fit.
Note that ultimately we'd like to have a single structure to hold all
the masks and other gubbins (per the very original intent of dma_parms),
so there's a fair chance of this getting fundamentally rejigged at some
point anyway.
Robin.
static inline int configure_dma_max_seg_size(struct device *dev)
{
if (!dev->dma_parms)
dev->dma_parms = kzalloc(sizeof(*dev->dma_parms), GFP_KERNEL);
if (!dev->dma_parms)
return -ENOMEM;
dma_set_max_seg_size(dev, DMA_BIT_MASK(32));
return 0;
}
static inline void clear_dma_max_seg_size(struct device *dev)
{
kfree(dev->dma_parms);
dev->dma_parms = NULL;
}
But this seems error prone and quite some code to add for every DMA
provider. So, I wondered if we couldn't have a helper for that. After
some brainstorming, I favour a dmam_-type of function. It will ensure
the memory gets freed and the pointer cleared on unbind. And it should
be easy to use.
I attached an RFC which I tested on a Renesas R-Car H3 SoC with the internal
DMAC of the SD controller. A branch can be found here (still waiting for
buildbot results):
git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git renesas/sdhi/set_max_seg
I added the companion function dmam_free_dma_parms() for completeness
although there is no user yet. I'd be totally open to drop it until
someone needs it.
Please let me know what you think. If this is the right track, I'll be
willing to fix the dangling pointers with it, too.
Thanks and happy hacking,
Wolfram
Wolfram Sang (2):
dma-mapping: introduce helper for setting dma_parms
mmc: sdhi: internal_dmac: set dma_parms
drivers/mmc/host/renesas_sdhi_internal_dmac.c | 2 +
include/linux/dma-mapping.h | 5 ++
kernel/dma/mapping.c | 50 +++++++++++++++++++
3 files changed, 57 insertions(+)
