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From: "Christian König" <christian.koenig@amd.com>
To: Alan Stern <stern@rowland.harvard.edu>
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 16:56:16 +0100	[thread overview]
Message-ID: <d2d581fb-ccba-00c9-0a22-b485870256ae@amd.com> (raw)
In-Reply-To: <20210219155328.GA1111829@rowland.harvard.edu>



Am 19.02.21 um 16:53 schrieb Alan Stern:
> 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.

Yeah, I was assuming something like that would work.

But in this case the USB device should give the host controllers device 
structure to the dma_buf_attach function so that the sg_table can be 
filled in with DMA addresses properly.

Regards,
Christian.

>
> Alan Stern

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WARNING: multiple messages have this Message-ID (diff)
From: "Christian König" <christian.koenig@amd.com>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Thomas Zimmermann <tzimmermann@suse.de>,
	daniel@ffwll.ch, airlied@linux.ie,
	maarten.lankhorst@linux.intel.com, mripard@kernel.org,
	sumit.semwal@linaro.org, gregkh@linuxfoundation.org,
	dri-devel@lists.freedesktop.org, noralf@tronnes.org,
	Christoph Hellwig <hch@lst.de>, Johan Hovold <johan@kernel.org>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	"Ahmed S. Darwish" <a.darwish@linutronix.de>,
	Mathias Nyman <mathias.nyman@linux.intel.com>,
	Oliver Neukum <oneukum@suse.com>, Felipe Balbi <balbi@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	stable@vger.kernel.org
Subject: Re: [PATCH] drm/prime: Only call dma_map_sgtable() for devices with DMA support
Date: Fri, 19 Feb 2021 16:56:16 +0100	[thread overview]
Message-ID: <d2d581fb-ccba-00c9-0a22-b485870256ae@amd.com> (raw)
In-Reply-To: <20210219155328.GA1111829@rowland.harvard.edu>



Am 19.02.21 um 16:53 schrieb Alan Stern:
> 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.

Yeah, I was assuming something like that would work.

But in this case the USB device should give the host controllers device 
structure to the dma_buf_attach function so that the sg_table can be 
filled in with DMA addresses properly.

Regards,
Christian.

>
> Alan Stern


  reply	other threads:[~2021-02-19 15:56 UTC|newest]

Thread overview: 28+ 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 13:40 ` Thomas Zimmermann
2021-02-19 13:45 ` Christian König
2021-02-19 13:45   ` Christian König
2021-02-19 14:34   ` Thomas Zimmermann
2021-02-19 14:34     ` Thomas Zimmermann
2021-02-19 14:43     ` Christian König
2021-02-19 14:43       ` Christian König
2021-02-19 14:43     ` Daniel Vetter
2021-02-19 14:43       ` Daniel Vetter
2021-02-19 15:53   ` Alan Stern
2021-02-19 15:53     ` Alan Stern
2021-02-19 15:56     ` Christian König [this message]
2021-02-19 15:56       ` Christian König
2021-02-19 16:41       ` Alan Stern
2021-02-19 16:41         ` Alan Stern
2021-02-22  7:52     ` Thomas Zimmermann
2021-02-22  7:52       ` Thomas Zimmermann
2021-02-20 13:02 ` Noralf Trønnes
2021-02-20 13:02   ` Noralf Trønnes
2021-02-21 20:56   ` Christian König
2021-02-21 20:56     ` Christian König
2021-02-22  7:54   ` Thomas Zimmermann
2021-02-22  7:54     ` Thomas Zimmermann
2021-02-22 17:30     ` Noralf Trønnes
2021-02-22 17:30       ` Noralf Trønnes
2021-02-23 10:14       ` Daniel Vetter
2021-02-23 10:14         ` Daniel Vetter

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