From: ". Christoph Hellwig" <hch@lst.de>
To: Ricardo Ribalda <ribalda@chromium.org>
Cc: Sergey Senozhatsky <senozhatsky@google.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Linux Doc Mailing List <linux-doc@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
IOMMU DRIVERS <iommu@lists.linux-foundation.org>,
Linux Media Mailing List <linux-media@vger.kernel.org>,
Mauro Carvalho Chehab <mchehab@kernel.org>,
Robin Murphy <robin.murphy@arm.com>,
". Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API
Date: Wed, 27 Jan 2021 16:56:08 +0100 [thread overview]
Message-ID: <20210127155608.GA20272@lst.de> (raw)
In-Reply-To: <CANiDSCsz+9DJesOTJ5C5HGEH-wwuTmEd3c8yLoHjnDz=2+ndJw@mail.gmail.com>
On Wed, Jan 27, 2021 at 12:29:08AM +0100, Ricardo Ribalda wrote:
> - Is there any platform where dma_alloc_noncontiguos can fail?
> This is, !ops->alloc_noncontiguous and !dev->coherent_dma_mask
> If yes then we need to add a function to let the driver know in
> advance that it has to use the coherent allocator (usb_alloc_coherent
> for uvc)
dev->coherent_dma_mask is set by the driver. So the only reason why
dma_alloc_noncontiguos will fail is because is because it can't
allocate any memory.
> - In dma_alloc_noncontiguos, on the dma_alloc_pages fallback. If we
> have a device where the dma happens in only one direction, could not
> get more performance with DMA_FROM/TO_DEVICE instead of
> DMA_BIDIRECTIONAL ?
Yes, we could probably do that.
>
>
> Then I have tried to use the API, and I have encountered a problem: on
> uvcvideo the device passed to the memory allocator is different for
> DMA_PAGES and NON_CONTIGUOUS:
> https://github.com/ribalda/linux/blob/042cd497739f71c8d4a83a67ee970369e2baca4a/drivers/media/usb/uvc/uvc_video.c#L1236
>
> I need to dig a bit tomorrow to figure out why this is, I have
> hardware to test both paths, so it should not be too difficult.
I always found the USB dma alloc API a little weird, but we might have
to follow the scheme of the usb coherent wrappers there.
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next prev parent reply other threads:[~2021-01-27 15:56 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-25 22:19 [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API Ricardo Ribalda
2020-11-26 11:44 ` Sergey Senozhatsky
2020-11-30 8:31 ` Christoph Hellwig
2020-11-30 8:34 ` Christoph Hellwig
2020-11-30 10:49 ` Ricardo Ribalda
2020-12-01 3:36 ` Sergey Senozhatsky
2020-12-01 14:49 ` Christoph Hellwig
2020-12-08 4:54 ` Tomasz Figa
2020-12-08 6:45 ` Sergey Senozhatsky via iommu
2020-12-08 7:13 ` Sergey Senozhatsky
2020-12-09 11:16 ` . Christoph Hellwig
2021-01-07 14:14 ` Ricardo Ribalda
2021-01-11 8:36 ` . Christoph Hellwig
2021-01-15 13:08 ` Ricardo Ribalda
2021-01-20 17:17 ` . Christoph Hellwig
2021-01-26 17:06 ` . Christoph Hellwig
2021-01-26 23:29 ` Ricardo Ribalda
2021-01-27 15:56 ` . Christoph Hellwig [this message]
2021-01-27 21:35 ` Ricardo Ribalda
2021-01-28 7:57 ` . Christoph Hellwig
2020-12-09 11:12 ` Christoph Hellwig
2020-12-09 13:05 ` Robin Murphy
2020-12-10 5:08 ` Tomasz Figa
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