On Thu, Jun 8, 2023 at 5:08 AM William Zhang william.zhang@broadcom.com wrote:
Due to the controller limitation to keep the chip select low during the bus idle time between the transfer, a dummy cs workaround was used when this driver was first upstreamed to the u-boot based on linux kernel driver. It basically picks the dummy cs as !actual_cs so typically dummy cs is 1 when most of the case only cs 0 is used in the board design. Then invert the polarity of both cs and tell the controller to start the transfers using dummy cs. Assuming both cs are active low before the inversion, effectively this keeps dummy cs high and actual cs low during the transfer and workaround the issue.
This workaround requires that dummy cs 1 pin to is set to SPI chip selection function in the pinmux when the transfer clock is above 25MHz. The old chips likely have default pinmux set to chip select on the dummy cs pin so it works but this is not case for the new Broadband BCA chips and this workaround stop working. This is specifically an issue to support SPI NAND and SPI NOR flash because these flash devices can typically run at or above 100MHz.
This patch utilizes the prepend feature of the controller to combine the multiple transfers in the same message to a single transfer when possible. This way there is no need to keep clock low between transfers and solve the issue without any pinmux requirement.
Multiple transfers within a SPI message may be combined into one transfer if the following are all true:
- One or more half duplex write transfer in single bit mode
- Optional full duplex read/write at the end
- No delay and cs_change between transfers
Most of the SPI device meets this requirements such as SPI NOR, SPI NAND flash, Broadcom SPI voice card and etc. So this change switches to the prepend mode as the default mode. For any SPI message that does not meet the above requirement, we switch to original dummy cs mode but limit the clock rate to the safe 25MHz.
Port from linux patch: Link: https://lore.kernel.org/r/20230209200246.141520-12-william.zhang@broadcom.co...
Signed-off-by: William Zhang william.zhang@broadcom.com
Acked-by: Jagan Teki jagan@amarulasolutions.com
Applied to u-boot-spi/master