On Sat, Jul 16, 2022 at 04:39:34PM +0100, Biju Das wrote: > On RZ/G2L SoCs switching from DMA to interrupt mode, causes timeout > issue as we are not getting Rx interrupt even though SPRF bit is set in > the status register. > > But there is no issue if we don't switch between interrupt to DMA mode > or vice versa. So we need this for correctness until someone can figure out what goes wrong :/ > Performance comparison between interrupt and DMA mode on RZ/Five SMARC > platform connected to a display module shows that performance and > CPU utilization is much better with DMA mode compared to interrupt mode > (1->65 fps) and (98->8%). How does that look for something doing lots of short transfers (like register I/O for example), and what's the throughput like? DMA probably is the most sensible default even so since the impact of doing PIO for large transfers tends to be so bad, you have to be doing a *lot* of small I/O before it gets to be a problem whereas basically any large transfer will notice PIO. > This patch introduces a variable force_dma to avoid switching between > DMA to interrupt mode for RZ platforms. Not that it really matters but it's not a variable, it's a flag in the device configuration. I'm wondering if we might want a way to override this at runtime in case someone does have an application that suffers badly from being forced into DMA (eg, some IIO thing), that could be done incrementally though.
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