> -----Original Message----- > From: Nori, Sekhar [mailto:[email protected]] > Sent: Thursday, January 14, 2010 12:44 AM > To: Brian Niebuhr > Cc: [email protected] > Subject: RE: DMA for Davinci SPI controller with multiple > slave devices > > Hello Brian, > > On Thu, Jan 14, 2010 at 04:26:15, Brian Niebuhr wrote: > > I am working on building a kernel for a custom board based > on an OMAP > > L138. As a baseline I am using the OMAP L138 SDK kernel. > On my board > > there are multiple SPI slaves connected to the SPI1 controller. I'm > > getting some errors during boot because after the first slave has > > initialized, the others report an error that they can't aquire a DMA > > channel. > > > > I've been looking through the code and I understand why the error > > occurs, but I'm not sure how to fix it correctly. Here's > what I know: > > > > - In arch/arm/mach-davinci/devices-da8xx.c there are two > DMA channels > > (one Tx, one Rx) allocated for the SPI1 controller. > > > > - In drivers/spi/davinci_spi_master.c the controller > maintains a struct > > davinci_spi_dma for every chip select. > > > > - In drivers/spi/davinci_spi_master.c, davinci_spi_probe() sets the > > dma_rx_sync_dev field of that structure for every chip > select to the Rx > > DMA channel, and likewise the dma_tx_sync_dev field of that > structure > > for every chip select to the Tx DMA channel. > > > > - Then for every SPI device, > > drivers/spi/davinci_spi_master.c:davinci_spi_setup() gets > called which > > in turn calls davinci_spi_request_dma(). > > > > - So here is the problem: davinci_spi_request_dma() requests the > > channels specified in dma_rx_sync_dev and dma_tx_sync_dev, > which are the > > same channels for every device. Therefore the first device > succeeds but > > additional devices on that controller fail. > > Yes, this was reported before when the driver patch was > submitted for review[1]. But, in the interest of having > the driver hit upstream sooner versus having all features > supported correctly, this has remained a TODO item. > > > > > > > So I'm trying to figure out what the driver writer intended so I can > > solve this problem. I see two potential solutions: > > > > 1. Allocate more DMA channels in devices-da8xx.c. Then in > > davinci_spi_probe() set the dma_[rx|tx]_sync_dev fields to > a different > > channel for each device. I don't fully understand though > if they would > > need separate eventqs too. This seems unnecessary, though, > because only > > one can be active at a time anyway. > > > > 2. Share the DMA channels between all devices on a controller. This > > seems like it would work fine except for the fact that the > DMA callbacks > > would be messed up as written. However I also think that > it would be > > possible to rewrite the callback function to work correctly. > > Right. Without a fix of some sort, the only other way is to not > to use DMA at all and use PIO mode instead. > > > > > > > I'm leaning toward (2), but I don't fully understand the driver. > > I believe (2) is the right way too. > > > Does > > anyone have any suggestions on how to fix this, or am I > just completely > > misunderstanding how this is supposed to work? > > A fix is needed. It would be great if this > is something you can help on.
I will attempt a fix, though it's going to be a while before I get to test it. If I can test before anyone else gets around to fixing it, I'll submit a patch. Brian > > Thanks, > Sekhar > > [1] > http://linux.omap.com/pipermail/davinci-linux-open-source/2009 > -November/016979.html > > > _______________________________________________ Davinci-linux-open-source mailing list [email protected] http://linux.davincidsp.com/mailman/listinfo/davinci-linux-open-source
