From: Alan Stern <stern@rowland.harvard.edu>
To: "Christian König" <christian.koenig@amd.com>
Cc: Felipe Balbi <balbi@kernel.org>,
Mathias Nyman <mathias.nyman@linux.intel.com>,
Thomas Zimmermann <tzimmermann@suse.de>,
airlied@linux.ie, gregkh@linuxfoundation.org,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Oliver Neukum <oneukum@suse.com>, Johan Hovold <johan@kernel.org>,
Christoph Hellwig <hch@lst.de>,
dri-devel@lists.freedesktop.org,
"Ahmed S. Darwish" <a.darwish@linutronix.de>,
stable@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject: Re: [PATCH] drm/prime: Only call dma_map_sgtable() for devices with DMA support
Date: Fri, 19 Feb 2021 10:53:28 -0500 [thread overview]
Message-ID: <20210219155328.GA1111829@rowland.harvard.edu> (raw)
In-Reply-To: <02a45c11-fc73-1e5a-3839-30b080950af8@amd.com>
On Fri, Feb 19, 2021 at 02:45:54PM +0100, Christian König wrote:
> Well as far as I can see this is a relative clear NAK.
>
> When a device can't do DMA and has no DMA mask then why it is requesting an
> sg-table in the first place?
This may not be important for your discussion, but I'd like to give an
answer to the question -- at least, for the case of USB.
A USB device cannot do DMA and has no DMA mask. Nevertheless, if you
want to send large amounts of bulk data to/from a USB device then using
an SG table is often a good way to do it. The reason is simple: All
communication with a USB device has to go through a USB host controller,
and many (though not all) host controllers _can_ do DMA and _do_ have a
DMA mask.
The USB mass-storage and uas drivers in particular make heavy use of
this mechanism.
Alan Stern
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel
next prev parent reply other threads:[~2021-02-19 16:00 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-19 13:40 [PATCH] drm/prime: Only call dma_map_sgtable() for devices with DMA support Thomas Zimmermann
2021-02-19 13:45 ` Christian König
2021-02-19 14:34 ` Thomas Zimmermann
2021-02-19 14:43 ` Christian König
2021-02-19 14:43 ` Daniel Vetter
2021-02-19 15:53 ` Alan Stern [this message]
2021-02-19 15:56 ` Christian König
2021-02-19 16:41 ` Alan Stern
2021-02-22 7:52 ` Thomas Zimmermann
2021-02-20 13:02 ` Noralf Trønnes
2021-02-21 20:56 ` Christian König
2021-02-22 7:54 ` Thomas Zimmermann
2021-02-22 17:30 ` Noralf Trønnes
2021-02-23 10:14 ` Daniel Vetter
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=20210219155328.GA1111829@rowland.harvard.edu \
--to=stern@rowland.harvard.edu \
--cc=a.darwish@linutronix.de \
--cc=airlied@linux.ie \
--cc=andriy.shevchenko@linux.intel.com \
--cc=balbi@kernel.org \
--cc=bigeasy@linutronix.de \
--cc=christian.koenig@amd.com \
--cc=dri-devel@lists.freedesktop.org \
--cc=gregkh@linuxfoundation.org \
--cc=hch@lst.de \
--cc=johan@kernel.org \
--cc=mathias.nyman@linux.intel.com \
--cc=oneukum@suse.com \
--cc=stable@vger.kernel.org \
--cc=tglx@linutronix.de \
--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