On Tue, 2017-10-31 at 17:09 +0530, Vinod Koul wrote:
> On Mon, Oct 30, 2017 at 06:39:07PM +0530, Shevchenko, Andriy wrote:
> > Per channel DMA parameters structure and sg_list (re-)split
> > -----------------------------------------------------------
> > Potentially some of DMA controllers may have different configuration
> > options on per channel basis. Inside struct device we have embedded
> > struct dma_parms which provides two fields: 1) maximum segment size,
> > and
> > 2) alignment boundary. API to set and retrieve these parameters is
> > bound to struct device. We might need something similar for struct
> > dma_chan.
>
> Sorry am not sure I follow, which parameters are we talking about
> here? Is
> it for dma_slave_caps?
Currently there is a struct device_dma_parameters in device.h which is
embedded in struct device.
Lars also pointed me to struct dma_slave_caps, so, my idea is to provide
parameters based on channel, not an entire device.
struct dma_slave_caps might be even more suitable for that.
But the former one has been added back to 2008 by commit 6b7b65105522
("iommu sg merging: add device_dma_parameters structure").
> > (That's why I stopped doing my patch series with sg_list split for
> > DMA
> > controller drivers)
> >
> > Some frameworks (SPI core, for example) are already using this to
> > prepare sg_list accordingly.
> >
> > The benefit of the change is to decrease latency for DMA setup when
> > sg_list is prepared without actual knowledge about DMA channel
> > parameters.
> >
> > Alexey shared opinion that there is no rush to implement it because
> > there is no example of such device in practice.
> >
> > Lars has different view on this, pointing to flexibility of the
> > drivers
> > (drivers know for sure all limitations and advantages of certain DMA
> > controller) and necessity to do at least a check since not all
> > subsystems are supporting mentioned API.
--
Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Intel Finland Oy
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