On Mon, Nov 21, 2016 at 10:43:29AM +0000, Russell King - ARM Linux wrote: > On Mon, Nov 21, 2016 at 12:29:01PM +0200, Mika Westerberg wrote: > > On Fri, Nov 18, 2016 at 07:35:42PM +0000, Russell King - ARM Linux wrote: > > > Another mitigation would be to lower the I2C bus frequency on Juno from > > > 400kHz to 100kHz, so that there's 4x longer IRQ latency possible. > > > However, even that isn't going to be reliable - even going to 100kHz > > > isn't going to allow the above case to be solved - the interrupt is > > > delayed by around 2ms, and it takes about 1.4ms to send/receive 16 bytes > > > at 100kHz. (9 * 16 / (100*10^3)). > > > > > > So, I think all hope is lost for i2c-designware on Juno to cope with > > > reading the EDID from TDA998x reliably. > > > > :-( > > > > I wonder if we can get it work more reliably by using DMA (provided that > > there are DMA channels available for I2C in Juno)? That would allow the > > hardware to perform longer reads without relying on how fast the > > interrupt handler is able to empty the Rx FIFO. > > It would need to DMA to the Tx FIFO to keep it filled - it triggers the > stop condition when the Tx FIFO empties. From what I can see in the > driver, the Tx FIFO not only takes the data but also a "command" to tell > the hardware what to do. Yes, the command is either "read" or "write" (meaning even for Rx a write to the Tx FIFO is needed). > The Rx FIFO would also need DMA to avoid it overflowing due to high > interrupt latency. I guess for Rx we would need to supply a dummy buffer of "read" commands for the Tx FIFO in addition to reading bytes from Rx FIFO. But still that might help to improve reliability in case of Juno. > I don't know what state DMA is in on the Juno, or even whether it has > DMA - it has a PL330 DMA controller, but I see nothing in the DT files > making use of it. OK -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
