From: Christoph Hellwig <hch@lst.de>
To: Jonathan Lemon <jonathan.lemon@gmail.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
iommu@lists.linux-foundation.org, bpf@vger.kernel.org,
"Björn Töpel" <bjorn.topel@intel.com>,
"Christoph Hellwig" <hch@lst.de>,
"Magnus Karlsson" <magnus.karlsson@intel.com>
Subject: Re: [PATCH 1/4] dma-mapping: add a new dma_need_sync API
Date: Tue, 7 Jul 2020 17:14:40 +0200 [thread overview]
Message-ID: <20200707151440.GA23816@lst.de> (raw)
In-Reply-To: <20200707151109.qui5uzzzq4dihfie@bsd-mbp>
On Tue, Jul 07, 2020 at 08:11:09AM -0700, Jonathan Lemon wrote:
> > You need to check every mapping. E.g. this API pairs with a
> > dma_map_single/page call. For S/G mappings you'd need to call it for
> > each entry, although if you have a use case for that we really should
> > add a dma_sg_need_sync helper instea of open coding the scatterlist walk.
>
> My use case is setting up a pinned memory area, and caching the dma
> mappings. I'd like to bypass storing the DMA addresses if they aren't
> needed. For example:
>
> setup()
> {
> if (dma_need_sync(dev, addr, len)) {
> kvmalloc_array(...)
> cache_dma_mappings(...)
> }
>
>
> dev_get_dma(page)
> {
> if (!cache)
> return page_to_phys(page)
>
> return dma_cache_lookup(...)
>
>
>
> The reason for doing it this way is that the page in question may be
> backed by either system memory, or device memory such as a GPU. For the
> latter, the GPU provides a table of DMA addresses where data may be
> accessed, so I'm unable to use the dma_map_page() API.
dma_need_sync doesn't tell you if the unmap needs the dma_addr_t.
I've been think about replacing CONFIG_NEED_DMA_MAP_STATE with a runtime
for a while, which would give you exattly what you need. For now it
isn't very useful as there are very few configs left that do not have
CONFIG_NEED_DMA_MAP_STATE set.
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next prev parent reply other threads:[~2020-07-07 15:14 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-29 13:03 add an API to check if a streamming mapping needs sync calls Christoph Hellwig
2020-06-29 13:03 ` [PATCH 1/4] dma-mapping: add a new dma_need_sync API Christoph Hellwig
2020-07-06 19:42 ` Jonathan Lemon
2020-07-07 6:47 ` Christoph Hellwig
2020-07-07 15:11 ` Jonathan Lemon
2020-07-07 15:14 ` Christoph Hellwig [this message]
2020-06-29 13:03 ` [PATCH 2/4] xsk: replace the cheap_dma flag with a dma_need_sync flag Christoph Hellwig
2020-06-29 13:03 ` [PATCH 3/4] xsk: remove a double pool->dev assignment in xp_dma_map Christoph Hellwig
2020-06-29 13:03 ` [PATCH 4/4] xsk: use dma_need_sync instead of reimplenting it Christoph Hellwig
2020-06-29 13:39 ` add an API to check if a streamming mapping needs sync calls Björn Töpel
2020-06-29 14:22 ` Christoph Hellwig
2020-07-08 7:44 ` Christoph Hellwig
2020-07-08 9:01 ` Daniel Borkmann
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