Messages in this thread | | | Date | Fri, 17 Apr 2015 00:03:30 +0200 | From | Noralf Trønnes <> | Subject | Re: [PATCH] dmaengine: bcm2835: Add slave dma support |
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Den 16.04.2015 21:06, skrev Alexander Stein: > Hi Stefan, > > On Wednesday 15 April 2015, 21:00:26 wrote Stefan Wahren: >> Am 15.04.2015 um 11:56 schrieb Noralf Trønnes: >>> Add slave transfer capability to BCM2835 dmaengine driver. >>> This patch is pulled from the bcm2708-dmaengine driver in the >>> Raspberry Pi repo. The work was done by Gellert Weisz. >>> >>> Tested with the bcm2835-mmc driver from the same repo. >> why not with the upstream kernel? > I also looked at slave dma support, especially for use in mmc. It turns our that bcm2835-mmc is written more or less completly new. > Mainline linux uses sdhci "framework" which internally uses the SDMA and/or ADMA (both internal, to SD/MMC controller, DMA units) which can be supported by an SDHCI compatible controller. > AFAIK the SD/MMC controller in bcm2835 lacks both that is why the driver only uses PIO. I dunno if external DMA usage can so easily be integrated into the sdhci, I have my doubts.
I asked Jonathan Bell (Raspberry Pi) about why a new driver was made instead of extending sdhci-bcm2835.
On 10.04.2015 20:02, Jonathan Bell wrote: > Basically, it's impossible to integrate platform DMA channel support > within the SDHCI framework. The Arasan controller (and the Broadcom > MMCI controller) both use platform DMA channels to pump data to/from > the host FIFO. Our old "sdhci-bcm2708" driver basically hacked sdhci.c > to allow platform DMA support in a way that was guaranteed to cause > merge conflicts with every new kernel branch. The reasoning behind > creating an MMC-level driver was to minimise disruption of incorporating > platform DMA and to have additional control e.g. on sequencing of commands > that are known to have bugs/problems. There are drivers in the source > tree that are "SDHCI compliant" but have their own various idiosyncrasies > - e.g. : http://lxr.free-electrons.com/source/drivers/mmc/host/davinci_mmc.c > Implementing an MMC-level driver was the easiest way to incorporate all > our various bits of baggage (random necessary delays here, busy-wait there) > without disrupting the rest of the codebase. I agree that some functions > could just substitute the sdhci.c equivalents and deduplicate some of the code.
Stephen Warren made this comment on a previous attempt to upstream the bcm2835-mmc driver:
On Tue, 28 Oct 2014 19:55:20 -0600, Stephen Warren wrote: > On 10/28/2014 06:00 PM, Piotr Król wrote: > > This is driver for Arasan External Mass Media Controller provided in > > Raspberry Pi single board computer. > > We should not have multiple drivers for the same HW. The correct > approach would be to enhance the existing sdhci-bcm2835.c to support any > new features or bug-fixes embodied within this driver. Presumably that > way, you'd also end up with a lot of small feature patches, which would > make patch review easier. Consequently I haven't reviewed this patch much.
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