On 11/12/2015 08:26 PM, Andy Shevchenko wrote: > On Fri, Nov 13, 2015 at 3:11 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >> On 11/12/2015 07:41 PM, Andy Shevchenko wrote: >>> On Thu, Nov 12, 2015 at 10:22 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote: >>>> On 11/12/2015 02:57 PM, One Thousand Gnomes wrote: >>> >>>> An illustrative (kernel-space) example is the mess that is dmaengine_pause(). >>>> Some DMA implementations provide the means to stop and restart DMA without >>>> losing data and some DMA implementations do not. Unfortunately, some >>>> advertise they support dmaengine_pause() but only for lossy uses like audio. >>>> Because the api hides this, the query interface for pause support is >>>> useless. >>> >>> The DMA pause() call means only pause with possibility to resume. >>> There is a resume() call as well. Any driver which treats pause() as a >>> complete stop is buggy driver and should be fixed. >> >> How about pause _without_ the possibility to resume? >> >> https://groups.google.com/d/msg/linux.kernel/Abe0hfGcgsw/H0se55wC558J > > Briefly what I got from the thread that Russell shows similar view on > the API, so that's why he was objecting to add pause/resume calls for > a specific hardware. Not quite. That dmaengine driver (omap-dma) advertises that it supports pause() via dma_get_slave_caps(). And if you call it with a cyclic channel it will pause. However, if you call dmaengine_pause() with a slave channel it returns an error *because the hardware can't actually meet the criteria for dmaengine_pause()* which is pause()/resume() without data loss. IOW, there is no method of determining a priori if dmaengine_pause() will categorically fail for a given transfer type. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
