From: "Ruhl, Michael J" <michael.j.ruhl@intel.com>
To: Marek Szyprowski <m.szyprowski@samsung.com>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
"linux-samsung-soc@vger.kernel.org"
<linux-samsung-soc@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
David Airlie <airlied@linux.ie>,
Shane Francis <bigbeeshane@gmail.com>,
"stable@vger.kernel.org" <stable@vger.kernel.org>,
Thomas Zimmermann <tzimmermann@suse.de>,
Alex Deucher <alexander.deucher@amd.com>
Subject: RE: [PATCH v2] drm/prime: fix extracting of the DMA addresses from a scatterlist
Date: Mon, 30 Mar 2020 13:46:10 +0000 [thread overview]
Message-ID: <14063C7AD467DE4B82DEDB5C278E8663FFFC0C0E@fmsmsx107.amr.corp.intel.com> (raw)
In-Reply-To: <8a09916d-5413-f9a8-bafa-2d8f0b8f892f@samsung.com>
>-----Original Message-----
>From: dri-devel <dri-devel-bounces@lists.freedesktop.org> On Behalf Of
>Marek Szyprowski
>Sent: Sunday, March 29, 2020 5:56 AM
>To: Ruhl, Michael J <michael.j.ruhl@intel.com>; dri-
>devel@lists.freedesktop.org; linux-samsung-soc@vger.kernel.org; linux-
>kernel@vger.kernel.org
>Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>; David Airlie
><airlied@linux.ie>; Shane Francis <bigbeeshane@gmail.com>;
>stable@vger.kernel.org; Thomas Zimmermann <tzimmermann@suse.de>;
>Alex Deucher <alexander.deucher@amd.com>
>Subject: Re: [PATCH v2] drm/prime: fix extracting of the DMA addresses from
>a scatterlist
>
>Hi Michael,
>
<snip>
>> Is there an example of what the scatterlist would look like in this case?
>
>DMA framework or IOMMU is allowed to join consecutive chunks while
>mapping if such operation is supported by the hw. Here is the example:
>
>Lets assume that we have a scatterlist with 4 4KiB pages of the physical
>addresses: 0x12000000, 0x13011000, 0x13012000, 0x11011000. The total
>size of the buffer is 16KiB. After mapping this scatterlist to a device
>behind an IOMMU it may end up as a contiguous buffer in the DMA (IOVA)
>address space. at 0xf0010000. The scatterlist will look like this:
>
>sg[0].page = 0x12000000
>sg[0].len = 4096
>sg[0].dma_addr = 0xf0010000
>sg[0].dma_len = 16384
>sg[1].page = 0x13011000
>sg[1].len = 4096
>sg[1].dma_addr = 0
>sg[1].dma_len = 0
>sg[2].page = 0x13012000
>sg[2].len = 4096
>sg[2].dma_addr = 0
>sg[2].dma_len = 0
>sg[3].page = 0x11011000
>sg[3].len = 4096
>sg[3].dma_addr = 0
>sg[3].dma_len = 0
>
>(I've intentionally wrote page as physical address to make it easier to
>understand, in real SGs it is stored a struct page pointer).
>
>> Does each SG entry always have the page and dma info? or could you have
>> entries that have page information only, and entries that have dma info
>only?
>When SG is not mapped yet it contains only the ->pages and ->len
>entries. I'm not aware of the SGs with the DMA information only, but in
>theory it might be possible to have such.
>> If the same entry has different size info (page_len = PAGE_SIZE,
>> dma_len = 4 * PAGE_SIZE?), are we guaranteed that the arrays (page and
>addrs) have
>> been sized correctly?
>
>There are always no more DMA related entries than the phys pages. If
>there is 1:1 mapping between physical memory and DMA (IOVA) space, then
>each SG entry will have len == dma_len, and dma_addr will be describing
>the same as page entry. DMA mapping framework is allowed only to join
>entries while mapping to DMA (IOVA).
>
>> Just trying to get my head wrapped around this.
>
>Sure, I hope my explanation helps a bit.
That is a great example! Thank you very much for the explanation.
I was somehow seeing it as the dma side getting split and extended (rather
than consolidated) into more possible entries. This clarifies the issue for me.
Thanks!
Mike
>Best regards
>--
>Marek Szyprowski, PhD
>Samsung R&D Institute Poland
>
>_______________________________________________
>dri-devel mailing list
>dri-devel@lists.freedesktop.org
>https://lists.freedesktop.org/mailman/listinfo/dri-devel
next prev parent reply other threads:[~2020-03-30 13:46 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CGME20200327162330eucas1p1b0413e0e9887aa76d3048f86d2166dcd@eucas1p1.samsung.com>
2020-03-27 16:21 ` [PATCH v2] drm/prime: fix extracting of the DMA addresses from a scatterlist Marek Szyprowski
2020-03-27 18:31 ` Ruhl, Michael J
2020-03-28 18:36 ` Shane Francis
2020-03-29 9:55 ` Marek Szyprowski
2020-03-30 13:10 ` Robin Murphy
2020-03-30 13:46 ` Ruhl, Michael J [this message]
2020-03-28 7:18 ` Greg KH
2020-04-05 14:47 ` Alex Deucher
2020-04-05 16:40 ` Greg KH
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=14063C7AD467DE4B82DEDB5C278E8663FFFC0C0E@fmsmsx107.amr.corp.intel.com \
--to=michael.j.ruhl@intel.com \
--cc=airlied@linux.ie \
--cc=alexander.deucher@amd.com \
--cc=b.zolnierkie@samsung.com \
--cc=bigbeeshane@gmail.com \
--cc=dri-devel@lists.freedesktop.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-samsung-soc@vger.kernel.org \
--cc=m.szyprowski@samsung.com \
--cc=stable@vger.kernel.org \
--cc=tzimmermann@suse.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).