[Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

Jerome Glisse j.glisse at gmail.com
Fri Apr 29 10:19:58 EDT 2011

On Fri, Apr 29, 2011 at 9:42 AM, Joerg Roedel <joro at 8bytes.org> wrote:
> On Thu, Apr 28, 2011 at 03:37:00PM -0400, Jerome Glisse wrote:
>> As Jesse pointed out already, for performance reasons it's lot better
>> if you let the driver decide even if you have an iommu capable of
>> handling coherency for you. My understanding is that each time
>> coherency is asked for it trigger bus activities of some kind (i think
>> snoop is the term used for pci) this traffic can slow down both the
>> cpu and the device. For graphic driver we have a lot of write once and
>> use (once or more) buffer and it makes a lot of sense to have those
>> buffer allocated using uncached memory so we can tell the device (in
>> case of drm driver) that there is no need to trigger snoop activities
>> for coherency. So i believe the decision should ultimately be in the
>> driver side.
> Stupid question: Couldn't these write-once-read-often buffers just stay
> in the memory of the GPU instead of refetching them every time from main
> memory? Or is that necessary because of the limited space on some GPUs?
> Regards,
>        Joerg

We might be talking about several G of data, so using system is not
uncommon. Also when uploading data to GPU vram is better to let the
GPU do dma from system memory rather than having the CPU do memcpy.


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