On Fri, Jan 15, 2021 at 11:19 PM Al Cooper <alcooperx@xxxxxxxxx> wrote: > > Add a UART driver for the new Broadcom 8250 based STB UART. The new > UART is backward compatible with the standard 8250, but has some > additional features. The new features include a high accuracy baud > rate clock system and DMA support. > > The driver will use the new optional BAUD MUX clock to select the best > one of the four master clocks (81MHz, 108MHz, 64MHz and 48MHz) to feed > the baud rate selection logic for any requested baud rate. This allows > for more accurate BAUD rates when high speed baud rates are selected. > > The driver will use the new UART DMA hardware if the UART DMA registers > are specified in Device Tree "reg" property. The DMA functionality can > be disabled on kernel boot with the argument: > "8250_bcm7271.disable_dma=Y". > > The driver also set the UPSTAT_AUTOCTS flag when hardware flow control > is enabled. This flag is needed for UARTs that don't assert a CTS > changed interrupt when CTS changes and AFE (Hardware Flow Control) is > enabled. > > The driver also contains a workaround for a bug in the Synopsis 8250 > core. The problem is that at high baud rates, the RX partial FIFO > timeout interrupt can occur but there is no RX data (DR not set in > the LSR register). In this case the driver will not read the Receive > Buffer Register, which clears the interrupt, and the system will get > continuous UART interrupts until the next RX character arrives. The > fix originally suggested by Synopsis was to read the Receive Buffer > Register and discard the character when the DR bit in the LSR was > not set, to clear the interrupt. The problem was that occasionally > a character would arrive just after the DR bit check and a valid > character would be discarded. The fix that was added will clear > receive interrupts to stop the interrupt, deassert RTS to insure > that no new data can arrive, wait for 1.5 character times for the > sender to react to RTS and then check for data and either do a dummy > read or a valid read. Sysfs error counters were also added and were > used to help create test software that would cause the error condition. > The counters can be found at: > /sys/devices/platform/rdb/*serial/rx_bad_timeout_late_char > /sys/devices/platform/rdb/*serial/rx_bad_timeout_no_char Brief looking into the code raises several questions: - is it driver from the last decade? - why it's not using what kernel provides? - we have a lot of nice helpers: - DMA Engine API - BIT() and GENMASK() macros - tons of different helpers like regmap API (if you wish to dump registers via debugfs) Can you shrink this driver by 20-30% (I truly believe it's possible) and split DMA driver to drivers/dma (which may already have something similar there)? -- With Best Regards, Andy Shevchenko
