linux-kernel - Re: [PATCH 8/8] WIP: add a dma_alloc

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Message-ID: <20201110094123.GA25672@lst.de>
Date:   Tue, 10 Nov 2020 10:41:23 +0100
From:   Christoph Hellwig <hch@....de>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Christoph Hellwig <hch@....de>, Tomasz Figa <tfiga@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [PATCH 8/8] WIP: add a dma_alloc_contiguous API

On Tue, Nov 10, 2020 at 10:33:05AM +0100, Ricardo Ribalda wrote:
> Also FYI, I managed to boot an ARM device with that tree. But I could
> not test the uvc driver (it was a remote device with no usb device
> attached)
> 
> Hopefully I will be able to test it for real this week.
> 
> Any suggestions for how to measure performance difference?

I have to admit I don't know at all how uvc works.  But the main
problem with dma_alloc_coherent is that all access is uncached.  So
anything that does larger and/or many data transfers to and from it
will be glacially slow.  With the dma streaming API we still have to
pay for cache flushes, but only before and after the transfers, and
in many cases in a somewhat optimized fashion.